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STUDIES 


MORPHOLOGICAL LABORATORY 


IN THE 


UNIVERSITY OF CAMBRIDGE. 


EDITED BY 


F. M. BALFOUR, M.A., F.RS., , 0) 


FELLOW OF TRINITY COLLEGE, CAMBRIDGE. 


Vol. |. Parts I. Il. 


London: 

Cc. J. CLAY AND SON, 
CAMBRIDGE UNIVERSITY PRESS WAREHOUSE, 
AVE MARIA LANE. 

1884 


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CONTENTS OF PART I. 


*Mr F. M. Barrour and Mr A. Sepewrck.—On the Existence 
of a Head-kidney in the Embryo Chick and on some 
Points in the Development of the Miillerian Duct, 
Plates I. and IT. 


*Mr F. M. Batrour.—On the Early Development of the Lacer- 
tilia, together with some Observations on the Nature 
and Relations of the Primitive Streak. Plate IIT. 


**Mr F. M. Batrour.—On Certain Points in the Anatomy of 
Peripatus Capensis 


*Mr W. B. Scorr and Mr Henry F. Osporn.—On some Points 
in the Early Development of the Common Newt . 


*Mr Apam Sepewicx.—Development of the Kidney in its 
Relation to the Wolffian Body in the Chick 


*Mr F. M. Batrour.—Notes on the Development of the 
Araneina . ; 


**Mr Apam Sepewick.—On the Development of the Struc- 


ture known as the ‘ Glomerulus of the Head-kidney’ in 
the Chick 


PAGE 


21 


31 


34 


62 


83 


107 


N.B.—The papers marked with one asterisk are reprinted from 
the ‘Quarterly Journal of Microscopical Science,’ those with two 
asterisks, are reprinted from the ‘Proceedings of the Cambridge 


Philosophical Society.’ 


CONTENTS OF PART IL. 


*Mr Sypney J. Hicxson.—The Eye of Pecten. Plates I. 
and II. 


*Mr Apam Srepewick.—On the Early Development of the An- 
terior Part of the Wolffian Duct and Body in the Chick, 
together with some Remarks on the Excretory System 
of the Vertebrata. Plate III. 


*** Mr F. M. Batrour.—On the Development of the Skeleton 
of the Paired Fins of Elasmobranchii, considered in re- 
lation to its bearings on the Nature of the Limbs of 
the Vertebrata. Plates IV.and V. . ; ‘ ‘ 


*Mr F. M. Batrour.—On the nature of the Organ in Adult 
Teleosteans and Ganoids, which is usually regarded as 
the Head Kidney or Pronephros 


*Mr K. Mirsuxuri.—On the Development of the Suprarenal 
Bodies in Mammalia. Plate VI. 


**Vr F. M. Batrour and W. N. Parxer.—On the Structure 
and Development of Lepidosteus 


**Mr ApAm Sepewick.—On Certain Points in the Anatomy 
of Chiton 


**Mr Watter Heare.—On the Germinal Layers and Early 
Development of the Mole 


*Mr F. M. Batrour and Mr F. Deicuton.—A Renewed 
Study of the Germinal Layers of the Chick. Plates 
VII. VIII. and IX. ; ‘ : ; 


PAGE 


1 


13 


51 


69 


75 


89 


99 


107 


117 


N.B.—The papers marked with one asterisk are reprinted from 
the ‘Quarterly Journal of Microscopical Science,’ the papers with 
two asterisks are reprinted from the ‘Proceedings of the Royal 
Society,’ and the paper with three asterisks from the ‘ Proceedings of 


the Zoological Society.’ 


SU DGS 


FROM THE 


MORPHOLOGICAL LABORATORY 


IN THE 


UNIVERSITY OF CAMBRIDGE, 


EDITED BY 


¥F. M. BALFOUR, M.A., F.R.S., 


FELLOW OF TRINITY COLLEGE, CAMBRIDGE. 


WILLIAMS AND NORGATE, 
14, HENRIETTA STREET, COVENT GARDEN, LONDON; 
anp 20, SOUTH FREDERICK STREET, EDINBURGH. 


1880. 


LONDON : 
PRINTED BY J. Kk. ADLARD, BARTHOLOMEW CLOSE. 


DEDICATED 
TO 


MICHAEL FOSTER. 


CONTENTS. 


*Mr. F. M. Batrour, and Mr. A. Sepe¢wick.—On the 
Existence of a Head-kidney in the Embryo Chick 
and on some Points in the Development of the 
Millerian Duct. Plates I and II 


*Mr. F. M. Batrour.—On the Early Development of the 
Lacertilia, together with some Observations on 
the Nature and Relations of the Primitive Streak. 
Plate III . 


** MR. FE. M. Batrour.—On estan hae in fie Ana- 
tomy of Peripatus Capensis 


*VMr. W. B. Scort and Mr. Henry F. Ospornn—On some 
Points in the Early Development of the Common 
Newt ; : 


*Mr. Apam Srpewicxk—Development of the Kidney in its 
Relation to the Wolffian Body in the Chick 


*Mr. F. M. Batrrour—Notes on the Poy conmey of the 
Araneina . 


**Mr. ADAM Santee 04 the eg oan of the 
Structure known as the ‘Glomerulus of the 
Head-kidney’ in the Chick 


PAGE 


21 


83 


107 


N.B.—The papers marked with one asterisk are reprinted from 
the ‘ Quarterly Journal of Microscopical Science,’ those with two 
asterisks, are reprinted from the ‘ Proceedings of the Cambridge 


Philosophical Society.’ 


On the Existmnce of a Huap-Kipnry in the Eupryo 
Cuick, and on Certain Points in the DnvELOPMENT 
of the Muutzrtan Duct. By F. M. Batrour, 
M.A., Fellow of Trinity College, Cambridge; and 
Apam Sepewick, B.A., Scholar of Trinity College, 
Cambridge. (With Plates I and IT.) 


Tue following paper is divided into three sections. The first 
of these records the existence of certain structures in the embryo 
chick, which eventually become in part the abdominal opening 
of the Millerian duct, and which, we believe, correspond with the 
head-kidney, or “‘ Vorniere’’ of German authors. The second 
deals with the growth and development of the Millerian duct. 
With reference to this we have come to the conclusion that the 
Miillerian duct does not develop entirely independently of the 
Wolffian duct. The third section of our paper is of a more 
general character, and contains a discussion of the rectifications 
in the views of the homologies of the parts of the excretory 
system in Aves, necessitated by the results of our investigations. 

We have, as far as possible, avoided entering into the ex- 
tended literature of the excretory system, since this has been 
very fully given in three general papers which have recently 
appeared by Semper,! Fiirbinger,” and by one of us.° 

All recent observers, including Braun* for Reptilia, and Egh? 
for Mammalia, have stated that the Miillerian duct develops as 
a groove in the peritoneal epithelium, which is continued back- 
ward as a primitively solid rod in the space between the Wolffian 
duct and peritoneal epithelium. 


' « Das Urogenital System der Plagiostomen.” ‘ Arbeiten a. d. Zool.- 
Zoot. Institut. Wirzburg.’ 

2 «Zur Vergl. Anat. u. Entwick. d. Excretionsorgane d. Vertebraten.” 
‘Morphologisches Jahrbuch,’ vol. iv. 

3 On the Origin and History of the Urino-genital Organs of Verte- 
brates.” ‘Journal of Anat. and Phys.,’ vol. x. 

4 « Arbeiten a. d. zool.-zoot. Institut. Wirzburg,’ vol. iv. 

§ «Beitr. zur Anat. u. Entwick. d. Geschlechtsorgane,’ Inaug. Diss. 
Zurich, 1876. 4 


4 1 


9 BALFOUR AND SEDGWICK, 


_ In our preliminary account we stated,' in accordance with the 
general view, that the Miillerian duct was formed as a groove, 
or elongated involution of the peritoneal epithelium adjoin- 
ing the Wolffian duct. We have now reason to believe that 
this is not the case. In the earliest condition of the Miillerian - 
duct which we have been able to observe, it consists of three suc- 
cessive open involutions of the peritoneal epithelium, connected 
together by more or less well-defined ridge-like thickenings of the 
epithelium. We believe, on grounds hereafter to be stated, that 
the whole of this formation is equivalent to the head-kidney of 
the Icthyopsida.- The head-kidney, as we shall continue to call 
it, takes its origin from the layer of thickened epithelium situated 
near the dorsal angle of the body cavity, close to the Wolffian 
duct, which has been known since the publication of Waldeyer’s 
important researches as the germinal epithelium. The anterior 
of the three open involutions or grooves is situated some little 
distance behind the front end of the Wolffian duct. It is simply 
a shallow groove in the thickest part of the germinal epithelium, 
and forms a corresponding projection into the adjacent stroma. 
In front the projection is separated by a considerable interval 
from the Wolffian duct; but near its hindermost part it 
almost comes into contact with the Wolffian duct. The groove 
extends in all for about five of our sections, and then terminates 
by its walls becoming gradually continued into a slight ridge- 
like thickening of the germinal epithelium. The groove arises 
as a simple depression in a linear area of thickened germinal 
epithelium. The linear area is, however, continued very con- 
siderably further forward than the groove, and sometimes exhibits 
a slight central depression, which might be regarded as a forward 
continuation of the groove. The passage from the groove to 
the ridge may best be conceived by supposing the groove to be 
suddenly filled up, so as to form a solid ridge pointing inwards 
towards the Wolffian duct. 

The ridge succeeding the first groove is continued for about 
six sections, and is considerably more prominent at its posterior 
extremity than in front. It is replaced by groove number two, 
which appears as if formed by the reverse process to that by 
which the ridge arose, viz., by a hollowing out of the ridge on 
the side towards the body cavity. The wall of the second 
groove is, after a few sections, continued into a second ridge or 
thickening of the germinal epithelium, which, however, is so 
faintly marked as to be hardly visible in its middle part. In its 
turn this ridge is replaced by the third and last groove. This 
vanishes after one or two sections, and behind the point of its 
disappearance we have failed to find any further traces of the 

' «Proceedings of Royal Society,’ 1878. 


> 


EXISTENCE O} HEAD-KIDNEY IN THE EMBRYO CHICK, 3 


head-kidney. The whole formation extends through about 
twenty-four of our sections and one and a half segments (muscle- 
lates). . 

: We have represented (Plate I, series a, Nos. 1—10) a fairly 
complete series of sections through part of the head-kidney of 
an.embryo slightly older than that last described, containing 
the second and third grooves and accessory parts. ~The connec- 
tion between the grooves and the ridges is very well illustrated in 
Nos. 3, 4, and 5, of this series. In No. 3 we have a pro- 
minent ridge, in the interior of which there appears in No. 4 
a groove, which becomes gradually wider in Nos. 5 and 6. 
Both the grooves and ridges are better marked in this than in 
the younger stage; but the chief difference between the two 
stages consists in the third groove no longer forming the hin- 
dermost limit of the head-kidney. Instead of this, the last 
groove (No. 7) terminates by the upper part of its walls becoming 
constricted off as a separate rod, which appears at first to contain 
a lumen continuous with the open groove. ‘This rod (Nos. 7, 8, 
9, 10) situated between the germinal epithelium and Wolffian 
duct is continued backward for some sections. It finally termi- 
- nates by a pointed extremity, composed of not more than two 
cells abreast (Nos. 8—10). 

Our third stage, sections of which are represented in series B 
(Plate I), is considerably advanced beyond that last described. 
The most important change which has been effected concerns 
the ridges connecting the successive grooves. A lumen has 
appeared in each of these, which seems to open at both ends 
into the adjacent grooves. At the same time the cells, which 
previously constituted the ridge, have become (except where they 
are continuous with the walls of the grooves) partially con- 
stricted off from the germinal epithelium. The ridges, in fact, 
now form ducts situated in the stroma of the ovarian ridge, in 
the space between the Wolffian duct and the germinal epithe- 
lum. The duct continuous with the last groove is somewhat 
longer than before. In a general way, the head-kidney may now 
be described as a duct opening into the body cavity by three 
groove-like apertures, and continuous behind with the rudiment 
of the true Miillerian duct. Although the general constitution 
of the head-kidney at this stage is fairly simple, there are a few 
features in our sections which we do not fully understand, 
and a few points about the organ which deserve a rather fuller 
description than we have given in this general sketch. 

The anterior groove (No. 1—3, series B, Pl. 1) is at first 
somewhat separated from the Wolffian duct, but’ approaches 
close to it in No. 3. In Nos. 2 and 3 there appears a rod-like 
body on the outer side of the walls of the groove, In No. 2 


4, BALFOUR AND SEDGWICK. 


this body is disconnected with the walls of the groove, and even 
appears as if formed by a second invagination of the germinal 
epithelium. In No. 3 this body becomes partially continuous 
with the walls of the groove, and finally in No. 4 it becomes 
completely continuous with the walls of the groove, and its lumen 
communicates freely with the groove.! 

The last trace of this body is seen on the upper wall of the 
groove in No. 5. We believe that the body (7,) represents the 
ridge between the first and second grooves of the earlier stage ; 
so that in passing from No. 3 to No. 5 we pass from the first to 
the second groove. The meaning of the features of the body (7,) 
in No. 2 we do not fully understand, but cannot regard them as 
purely accidental, since we have met with more or less similar 
features in other series of sections. The second groove becomes 

radually narrower, and finally is continued into the second ridge 
(No. 8). The ridge contains a lumen, and is only connected 
with the germinal epithelium by a narrow wall of cells. A 
narrow passage from the body cavity leads into that wall for a 
short distance in No. 8, but it is probably merely the hinder end 
of the groove of No. 7. The third groove appears in No. 11, 
and opens into the lumen of the second ridge (7.) in No. 12. In 
No. 13 the groove is closed, and there is present in its place a 
duct (r,) connected with the germinal epithelium by a wall of 
cells. This duct is the further development of the third ridge of 
the last stage ; its lumen opens into the body cavity through the 
third and last groove (gr). In the next section this duct (7 ,) 
is entirely separated from the germinal epithelium, and it may 
be traced backwards through several sections until it terminates 
by a solid point, very much as in the last stage. 

In the figures of this series (B) there may be noticed on 
the outer side of the Miillerian duct a fold of the germinal 
epithelium (#) forming a second groove. It is especially con- 
spicuous in the first six sections of the series. This fold some- 
times becomes much deeper, and then forms a groove, the upper 
end of which is close to the grooves of the head-kidney. It is 
very often much deeper than these are, and without careful study 
might easily be mistaken for one of these grooves. Fig. co, 
taken from a series slightly younger than B, shows this groove 
(x) in its most exaggerated form. 

The stage we have just described is that of the fullest develop- 
ment of the head-kidney. In it, as in all the previous stages, 
there appear to be only three main openings into the body-cavity ; 
but we have met in some of our sections with indications of the 
possible presence of one or two extra rudimentary grooves. 


1 A deep focus of the rather thick section represented in No. 3 showed 
the body much more nearly in the position it occupies in No. 4. 


EXISTENCE OF HEAD-KIDNEY IN THE EMBRYO CHICK, a 


In an embryo not very much older than the one last described 
the atrophy of the head-kidney is nearly completed, and there is 
present but a single groove opening into the body cavity. _ 

In series p (Pl. IL) are represented a number of sections 
from an embryo at this stage. Nos. 1 and 2 are sections through 
the hind end of the single groove now present. Its walls are 
widely separated from the Wolffian duct in front, but approach 
close to it at the hinder termination of the groove (No. 2). The 
features of the single groove present at this stage agree closely 
with those of the anterior groove of the previous stages. The 
groove is continued into a duct—the Miillerian duct (as it may 
now be called, but in a previous stage the hollow ridge connecting 
the first and second grooves of the head kidney)—which, after 
becoming nearly separated from the germinal epithelium, is again 
connected to it by a mass of cells at two points (Nos. 5, 6, and 
8). The germinal epithelium is slightly grooved and is much 
reduced in thickness at these points of contact (gr. and gr), and 
we believe that they are the remnants of the posterior grooves of 
the head-kidney present at an earlier stage. 

The Miillerian duct has by this stage grown much further 
backwards, but the peculiarities of this part of it are treated in 
a subsequent section. 

We consider that, taking into account the rudiments we have 
just described, as well as the fact that the features of the single 
groove at this stage correspond with those of the anterior groove 
at an earlier stage, we are fully justified in concluding that the 
permanent abdominal opening of the Miillerian duct corresponds 
with the anterior of our three grooves. 

Although we have, on account of their indefiniteness, avoided 
giving the ages of the chicks in which the successive changes of 
the head-kidney may be observed, we may, perhaps, state that all 
the changes we have described are usually completed between 
the 90th and 120th hour of incubation. 


The Glomerulus of the Head-Kidney. 


In connection with the head-kidney in Amphibians there is 
present, as is well known, a peculiar vascular body usually de- 
scribed as the glomerulus of the head-kidney. We have found 
in the chick a body so completely answering to this glomerulus 
that we have hardly any hesitation in identifying it as such. 

In the chick the glomerulus is paired, and consists of a vascular 
outgrowth or ridge projecting into the body cavity on each side 
at the root of the mesentery. It extends from the anterior end 
of the Wolffian body to the point where the foremost opening 
of the head-kidney commences. We have found it at a period 
slightly earlier than that of the first development of the head- 


6 BALFOUR AND SEDGWICK. 


kidney. It is represented in figs. z and F, Pl. II g/, and is 
seen to form a somewhat irregular projection into the body 
cavity, covered by a continuation of the peritoneal epithelium, and 
attached by a narrow stalk to the insertion of the embryonic 
mesentery (7e). 

In the interior of this body is seen a stroma with numerous 
vascular channels and blood-corpuscles, and a vascular connec- 
tion is apparently becoming established, if it is not so already, 
between the glomerulus and the aorta. We have reason to think 
that the corpuscles and vascular channels in the glomerulus are 
developed im situ. The stalk connecting the glomerulus with 
the attachment of the mesentery varies in thickness in different 
sections, but we believe that the glomerulus is continued un- 
broken throughout the very considerable region through which 
it extends. This point is, however, difficult to make sure of 
owing to the facility with which the glomerulus breaks away. 

At the stage we are describing, no true Malpighian bodies are 
present in the part of the Wolffian body on the same level 
with the anterior end of the glomerulus, but the Wolffian body 
merely consists of the Wolffian duct. At the level of the pos- 
_ terior part of the glomerulus this is no longer the case, but here 
a regular series of primary Malpighian bodies is present (using 
the term “ primary ”’ to denote the Malpighian bodies developed 
directly out of part of the primary segmental tubes), and the 
glomerulus of the head-kidney may frequently be seen in the same 
section as a Malpighian body. In most sections the two bodies 
appear quite disconnected, but in those sections in which the 
glomerulus of the Malpighian body comes into view it is seen to 
be derived from the same formation as the glomerulus of the 
head-kidney (Plate II, fig. Fr). It would seem, in fact, that the 
vascular tissue of the glomerulus of the head-kidney grows into the 
concavity of the Malpighian bodies. Owing to the stage we are 
now describing, in which we have found the glomerulus most fully 
developed, being prior to that in which the head-kidney appears, © 
it is not possible to determine with certainty the position of the 
glomerulus in relation to the head-kidney. After the develop- 
ment of the head-kidney it is found, however, as we have already 
stated, that the glomerulus terminates at a point just in front of 
the anterior opening of the head-kidney. It is less developed 
than before, but is still present up to the period of the atrophy 
of the head-kidney. It does not apparently alter in constitu- 
tion, and we have not thought it worth while giving any further 
representations of it during the later stages of its existence. 

Summary of the development of the head-kidney and glomerulus. 
—The first rudiment of the head-kidney arises as three successive 
grooves in the thickened germinal epithelium, connected by ridges 


EXISTENCE OF HEAD-KIDNEY IN THE EMBRYO CHICK, 7 


and situated some way behind the front end of the Wolffian duct. 
In the next stage the three ridges connecting the grooves have 
become more marked, and in each of them a lumen has appeared, 
opening at both extremities into the adjoining grooves. Still later 
the ridges become more or less completely detached from the 
peritoneal epithelium, and the whole head-kidney then consists 
of a slightly convoluted duct, with, at the least, three peritoneal 
openings, which is posteriorly continued into the Miillerian duct. 
Still later the head-kidney atrophies, its two posterior openings 
vanishing, and its anterior opening remaining as the permanent 
opening of the Miillerian duct. The glomerulus arises as a 
vascular prominence at the root of the mesentery, slightly prior 
in point of time to the head-kidney, and slightly more forward 
than it in position. We have not traced its atrophy. 

We stated in our preliminary paper that the peculiar struc- 
tures we had interpreted as the head-kidney had completely 
escaped the attention of previous observers, though we called 
attention to a well-known figure of Waldeyer’s (copied in the 
‘Elements of Embryology,’ fig. 51).- In this figure a connec- 
tion between the germinal epithelium and the Millerian duct is 
drawn, which is probably part of the head-kidney, and may be 
compared with our figures (Series B, No. 8, and Series p, No. 4). 
Since we made’the above statement, Dr. Gasser has called our 
attention to a passage in his valuable memoir on ‘ The Develop- 
ment of the Allantois,’4in which certain structures are described 
which are, perhaps, identical with our head-kidney. The fol- 
lowing is a translation of the passage : 

“Jn the upper region of Miiller’s duct I have often observed 
small canals, especially in the later stages of development, which 
appear as a kind of doubling of the duct, and run for a short 
distance close to Miiller’s duct and in the same direction, open- 
ing, however, into the body cavity posterior to the main duct. 
Further, one may often observe diverticula from the extreme 
anterior end of the oviduct of the bird, which form blind pouches 
and give one the impression of being receptacula seminis. Both 
these appearances can quite well be accounted for on the supposi- 
tion that an abnormal communication is effected between the 
germinal epithelium and Miiller’s duct at unusual places; or 
else that an attempt at such a communication is made, resulting, 
however, only in the formation of a diverticulum of the wall of 
the oviduct.” 

The statement that these accessory canals are late in developing, 
prevents us from feeling quite confident that they really cor- 
respond with our head-kidney. 


1 * Beitrage zur Entwick lungsgeschichte d, Allantois der Miiller’schen 
Gange u, des Afters, Frankfurt, 1874, 


8 BALFOUR AND SEDGWICK. 


Before passing on to the other parts of this paper it is necessary 
to say a few words in justification of the comparison we have 
made between the modified abdominal extremity of the Mullerian 
duct in the chick and the head-kidney of the Icthyopsida. 

For the fullest statement of what is known with reference to the 
anatomy and development of the head-kidney in the lower types 
we may refer to Spengel and Fiirbringer.' We propose ourselves 
merely giving a sufficient account of the head-kidney in Amphibia 
(which appears to be the type in which the head-kidney can be 
most advantageously compared with that in the bird) to bring 
out the grounds for our determination of the homologies. 

The development of the head-kidney in Amphibia has been 
fully elucidated by the researches of W. Miiller,” Gdotte,* and 
Fiirbringer,* while to the latter we are indebted for a knowledge 
of the development of the Miillerian duct in Amphibians. The 
first part of the urino-genital system to develop is the segmental 
duct (Vornieregang of Fiirbringer), which is formed by a groove- 
like invagination of the-peritoneal epithelium. It becomes con- 
stricted into a duct first of allin the middle, but soon in the pos- 
terior part also. It then forms a duct, ending in front by a groove 
in free communication with the body cavity, and terminating 
blindly behind. The open groove in front at first deepens, and 
then becomes partially constricted into a duct, which elongates 
and becomes convoluted, but remains in communication with the 
body cavity by from two to four (according to the species) 
separate openings. ‘The manner in which the primitive single 
opening is related to the secondary openings 1s not fully under- 
stood. By these changes there is formed out of the primitive 
groove an anterior glandular body, communicating with the body 
cavity by several apertures, and a posterior duct, which carries 
off the secretion of the gland, and which, though blind at first, 
eventually opens into the cloaca. In addition to these parts there 
is also formed on each side of the mesentery, opposite the peri- 
toneal openings, a very vascular projection into this part of the 
body cavity, which is known as the glomerulus of the head- 
kidney, and which very closely resembles in structure and posi- 
tion the body to which we have assigned the same name in the 
chick, 

The primitive segmental duct is at first only the duct for 
the head-kidney, but on the formation of the posterior parts of 
the kidney (Wolffian body, &c.) it becomes the duct for these 
also. 

1 Loc. cit. . 

2 « Jenaische Zeitschrift,’ vol. ix, 1875. 
. sgh cosa ed ua d. Unke.’ 
* Loc, cit, 


EXISTENCE OF HEAD-KIDNEY IN THE EMBRYO CHICK. 9 


After the Wolffian bodies have attained to a considerable deve- 
lopment, the head-kidney undergoes atrophy, and its peritoneal 
openings become successively closed from before backwards. 
At this period the formation of the Miillerian duct takes place. 
It is a solid constriction of the ventral or lateral wall of the 
segmental duct, which subsequently becomes hollow, and ac- 
quires an opening into the body cavity guzte independent of the 
openings of the head-kidney. 

The similarity in development and structure between the 
head-kidney in Amphibia and the body we have identified as such 
in Aves, is to our minds too striking to be denied. Both consist 
of two parts—(1) a somewhat convoluted longitudinal canal, 
with a certain number of peritoneal openings; (2) a vascular 
prominence at the root of the mesentery, which forms a glo- 
merulus. As to the identity in position of the two organs we 
hope to deal with that more fully in speaking of the general 
structure of the excretory system, but may say that one of us! 
has already, on other grounds, attempted to show that the ab- 
dominal opening of the Miillerian duct in the bird is the homo- 
logue of the abdominal opening of the segmental ductin Amphibia, 
Elasmobranchii, &c., and that we believe that this homology will 
be admitted by most anatomists. If this homology is admitted, 
the identity in position of this organ in Aves and Amphibia 
necessarily follows. The most striking difference between Aves 
and Amphibia in relation to these structures is the fact that in 
Aves the anterior pore of the headskidney remains as the perma- 
nent opening of the Miillerian duct, while in Amphibia, the 
pores of the head-kidney atrophy, and an entirely fresh abdo- 
minal opening is formed for the Miillerian duct. 


ing 
The Growth of the Miillerian Duct. 


Although a great variety of views have been expressed by 
different observers on the growth of the Miillerian duct, it is 
now fairly generally admitted that it grows in the space between 
a portion of the thickened germinal epithelium and the Wolffian 
duct, but quite independently of both of them. Both Braun 
and Hgli, who have specially directed their attention to this 
point, have for Reptilia and Mammalia fully confirmed the views 
of previous observers. We were, nevertheless, induced, partly 
on account of the @ priori difficulties of this view, and partly by 
certain peculiar appearances which we observed, to undertake 

' Balfour, ‘Origin and History of Urinogenital Organs of Vertebrates.” 


‘Journal of Anat, and Phys.,’ vol. x, and ** Monograph on Elasmobranch 
Fishes,” 


10 -BALFOUR AND SEDGWICK. 


the re-examination of this point, and have found ourselves unable 
altogether to accept the general account. We propose first 
describing, in as matter-of-fact a way as possible, the actual 
observations we have made, and then stating what conclusions 
we think may be drawn from these observations. 

We have found it necessary to distinguish three stages in the 
growth of the Miullerian duct. Our first stage embraces the 
period prior to the disappearance of the head-kidney. At this 
stage, the structure we have already spoken of as the rudiment 
of the Miillerian duct consists of a solid rod of cells, continuous 
with the third groove of the head-kidney. It extends through a 
very few sections, and terminates by a fine point of about two 
cells, wedged in between the Wolffian duct and germinal epithe- 
lium (described above, No. 7—10, series a, Plate I). 

In an embryo slightly older than the above, such as that 
from which series B was taken, but still belonging to our first 
stage, a definite lumen appears in the anterior part of the 
Miillerian duct, which vanishes after a few sections. The duct 
terminates in a point which lies in a concavity of the wall of the 
Wolffian duct (Plate I, Nos. 1 and 2, series a). The limits 
of the Wolffian wall and the pointed termination of the Miller- 
ian duct are in many instances quite distinct; but the outline 
of the Wolffian duct appears to be carried round the Millerian 
duct, and im some instances the terminal point of the Millerian 
duct seems almost to form an integral part of the wall of the 
Wolffian duct. 

The second of our stages corresponds with that in which the 
atrophy of the head-kidney is nearly complete (series D and u, 
Plate II). 

The Miillerian duct has by this stage made a very marked 
progress in its growth towards the cloaca, and, in contradistinction 
to the earlier stage, a lumen is now continued close up to the 
terminal point of the duct. In the two or three sections before 
it ends it appears as a distinct oval mass of cells (No. 10, series 
p, and No. 1, series 4), without a lumen, lying between and 
touching the external wall of the Wolffian duct on the one hand, 
and the germinal epithelium on the other. It may either lie on 
the ventral side of the Wolffian duct (series D), or on the outer 
side (series H), but in either case is opposite the maximum 
thickening of that part of the germinal epithelium which always 
accompanies the Millerian duct in its backward growth. 

In the last section in which any trace of the Miillerian duct 
can be made out (series p, No. 11, and series u, No. 2), it 
has no longer an oval, well-defined contour, but appears to have 
completely fused with the wall of the Wolffian duct, which is 
accordingly very thick, and occupies the space which in the pre- 


EXISTENCE OF HEAD-KIDNEY IN THE EMBRYO CHICK, 11 


vious section was filled by its own wall and the Miillerian duct. 
In the following section the thickening in the wall of the 
Wolffian duct has disappeared (Plate II, series H, No. 8), and 
every trace of the Miillerian duct has vanished from view. 
The Wolffian duct is on one side in contact with the germinal 
epithelium. 

The stage during which the condition above described lasts 
is not of long duration, but is soon succeeded by our third 
stage, in which a fresh mode of termination of the Miillerian 
duet is found. (Plate II, series 1). This last stage remains 
up to about the close of the sixth day, beyond which our investi- 
gations do not extend. 

A typical series of sections through the terminal part of the 
Miillerian duct at this stage presents the following features : 

A few sections before its termination the Miillerian 
duct appears as a well-defined oval duct lying in contact 
with the wall of the Wolffian duct on the one hand 
and the germinal epithelium on the other (series 1, No. 1). 
Gradually, however, as we pass backwards, the Miillerian 
duct dilates; the external wall of the Wolffian duct adjoining it 
becomes greatly thickened and pushed in in its middle part, so 
as almost to touch the opposite wall of the duct, and so form a 
bay in which the Millerian duct lies (Plate II, series 1, Nos. 
2 and 3). Assoon as the Miillerian duct has come to lie in 
this bay its walls lose their previous distinctness of outline, 
and the cells composing them assume a curious vacuolated 
appearance. No well-defined line of separation can any longer 
be traced between the walls of the Wolffian duct and those of 
the Miillerian, but between the two is a narrow clear space 
traversed by an irregular network of fibres, in some of the 
meshes of which nuclei are present. 

The Millerian duct may be traced in this condition for a con- 
siderable number of sections, the peculiar features above de- 
scribed becoming more and more marked as its termination is 
approached. It continues to dilate and attains a maximum size 
in the section or so before it disappears. A lumen may be 
observed in it up to its very end, but is usually irregular in 
outline and frequently traversed by strands of protoplasm. The 
Miillerian duct finally terminates quite suddenly (Plate II, 
series 1, No. 4), and in the section immediately behind its ter- 
mination the Wolffian duct assumes its normal appearance, and 
the part of its outer wall on the level of the Millerian duct 
comes into contact with the germinal epithelium (Plate IT, 
series 1, No. 5). 

We have traced the growing point of the Miillerian duct with 
the above features till not far from the cloaca, but we have not 


12 BALFOUR AND SEDGWICK. 


followed the last phases of its growth and its final opening 
into the cloaca. 

In some of our embryos we have noticed certain rather pecu- 
liar structures, an example of which is represented at y in fig. k, 
taken from an embryo of 123 hours, in which all traces of the 
head-kidney had disappeared. It Consists of a cord of cells, 
connecting the Wolffian duct and the hind end of the abdominal 
opening of the Miillerian duct. At the least one similar cord 
was met with in the same embryo, situated just behind the ab- 
dominal opening of the Millerian duct. We have found similar 
structures in other embryos of about the same age, though never 
so well marked as in the embryo from which fig. kK is taken. 
We have quite failed to make out the meaning, if any, of them. 

Our interpretation of the appearances we have described in 
connection with the growth of the Miillerian duct can be stated 
in a very few words. Our second stage, where the solid point 
of the Miillerian duct terminates by fusing with the walls of 
the Wolffian duct, we interpret as meaning that the Miilerian 
is growing backwards as a solid rod of cells, split off from the 
outer wall of the Wolffian duct; in the same manner, in fact, as 
in Amphibia and Elasmobranchii. The condition of the terminal 
part of the Miullerian duct during our third stage cannot, we 
think, be interpreted in the same way, but the peculiarities of the 
cells of both Millerian and Wolffian ducts, and the indistinctness 
of the outlines between them, appear to indicate that the 
Miillerian duct grows by cells passing from the Wolffian duct 
to it. In fact, although in a certain sense the growth of the 
two ducts is independent, yet the actual cells which assist in 
the growth of the Miillerian duct are, we believe, derived from 
the walls of the Wolffian duct. 


att, 
General Considerations. 


The excretory system of a typical Vertebrate consists of the 
following parts: 


1. A head-kidney with the characters already described. 

2. A duct for the head-kidney—the segmental duct. 

3. A posterior kidney—(W olffian body, permanent kidney, &c. 
The nature and relation of these parts we leave out of considera- 
tion, as they have no bearing upon our present investigations.) 
The primitive duct for the Wolffian body is the segmental duct. 
_ 4, The segmental duct may become split into (a) a dorsal or 
inner duct, which serves as ureter (in the widest sense of the 
word) ; and (4) a ventral or outer duct, which has an opening 


EXISTENCE OF HEAD-KIDNEY IN THE EMBRYO CHICK. 13 


into the body cavity, and serves as the generative duct for the 
female, or for both sexes. 

These parts exhibit considerable variations both in their struc- 
ture and development, into some of which it is necessary for us 
to enter. 

The head-kidney! attains to its highest development in the 
Marsipobranchii (Myxine, Bdellostoma). It consists of a lon- 
gitudinal canal, from the ventral side of which numerous tubules 
pass. These tubules, after considerable subdivision, open by a 
large number of apertures into the pericardial cavity. From 
the longitudinal canal a few dorsal diverticula, provided with 
glomeruli, are given off. In the young the longitudinal canal 
is continued into the segmental duct ; but this connection becomes 
lost in the adult. The head-kidney remains, however, through 
life. In Teleostei and Ganoidei (?) the head-kidney is generally 
believed to remain through life, as the dilated cephalic portion of 
the kidneys when such is present. In Petromyzon and Amphi- 
bia the head-kidney atrophies. In Elasmobranchii the head- 
kidney, so far as is known, is absent. 

The.development of the segmental duct and head-kidney (when 
present) is still more important for our purpose than their adult 
structure. 

In Myxine the development of these structures is not known. 
In Amphibia and Teleostei it takes place upon the same type, 
viz., by the conversion of a groove-like invagination of the peri- 
toneal epithelium into a canal open in front. The head-kidney 
is developed from the anterior end of this canal, the opening of 
which remains in Teleostei single and closes early in embryonic 
life, but becomes in Amphibia divided into two, three, or four 
openings. In Elasmobranchii the development is very different. 

“The first trace of the urinary system makes its appearance as 
a knob springing from the intermediate cell-mass opposite the 
fifth proto-vertebra. . This knob is the rudiment of the abdominal 
opening of the segmental duct, and from it there grows backwards 
to the level of the anus a solid column of cells, which constitutes 
the rudiment of the segmental duct itself. The knob projects 

1 T am inclined to give up the view I formerly expressed with reference 
to the head-kidney and segmental duct, viz. “ that they were to be regarded 
as the most anterior segmental tube, the peritoneal opening of which had 
become divided, and which had become prolonged backwards so as to serve 
as the duct for the posterior segmental tubes,” and provisionally to accept 
the Gegenbaur-Fiirbringer view which has been fully worked out and ably 
argued for by Fiirbringer (loc. cit. p. 96). According to this view the head- 
kidney and its duct are to be looked on as the primitive and unsegmented 
part of the excretory system, more or less similar to the excretory system 
of many Trematodes and unsegmented Vermes. The segmental tubes | 


regard as a truly segmental part of the excretory system acquired subse- 
quently. —F. M. B. 


14. BALFOUR AND SEDGWICK. 


towards the epiblast, and the column connected with it hes 
between the mesoblast and epiblast. The knob and column do 
not long remain solid, but the former acquires an opening into 
the body-cavity continuous with a lumen, which makes its appear- 
ance in the latter.’””! 

The difference in the development of the segmental duct in the 
two types (Amphibia and Elasmobranchii) is very important. In 
the one case a continuous groove of the peritoneal epithelium 
becomes constricted into a canal, in the other a solid knob of 
cells is continued into a rod, at first solid, which grows backwards 
without any apparent relation to the peritoneal epithelium. 

The abdominalaperture ofthe segmental ductin Elasmobranchu, 
in that it becomes the permanent abdominal opening of the ovi- 
duct, corresponds physiologically rather with the abdominal open- 
ing of the Millerian duct than with that of the segmental duct of 
Amphibia, which, after becoming divided up to form the pores of 
the head-kidney, undergoes atrophy. Morphologically, however, 
it appears to correspond with the opening of the segmental duct 
in Amphibia. We shall allude to this point more than once again, 
and give our grounds for the above view on p. 19. 

The development of the segmental duct in Elasmobranchii asa 
solid rod is, we hope to show, of special importance for the 
elucidation of the excretory system of Aves. | 

The development of these parts Petromyzon is not fully 
known, but from W. Miiller’s account (‘ Jenaische Zeitschrift,’ 
1875) it would seem that an anterior invagination of the peri- 

1Tn a note on p. 50 of his memoir Firbringer criticises my description of 
the mode of growth of the segmental duct. The following is a free trans- 
lation of what he says: “In Balfour’s, as in other descriptions, an account 
is given of a backward growth, which easily leads to the supposition of a 
structure formed anteriorly forcing its way through the tissues behind. 
This is, however, not the case, since, to my knowledge, no author has ever 
detected a sharp boundary between the growing point of the segmental 
duct (or Miillerian duct) and the surrounding tissues.” He goes on to 
say that ‘“‘the growth in these cases really takes place by a differentiation 
of tissue along a line in the region of the peritoneal cavity.”? Although I 
fully admit that it would be far easier to homologise the development of the 
segmental duct in Amphibia and Hlasmobranchti according to this view, I 
must nevertheless vindicate the accuracy of my original account. I have 
looked over my specimens again, since the appearance of Dr. Fiirbringer’s 
paper, and can find no evidence of the end of the duct becoming continuous 
with the adjoining mesoblastic tissues. In the section, before its dis- 
appearance, the segmental duct may, so far as I can make out, be seen as a 
very small but distinct rod, which is much more closely connected with the 
epiblast than with any other layer. From Gasser’s observations on the 
Wolffian duct in the bird, I am led to conclude that it behaves in the same 
way as the segmental duct in the Hlasmobranchii. I will not deny that it is 
possible that the growth of the duct takes place by wandering cells, but on 


this point I have no evidence, and must therefore leave the question an 
open one.—I’, M. B, 


EXISTENCE OF HEAD-KIDNEY IN THE EMBRYO CHICK. 15 , 


toneal epithelium is continued backwards as a duct (segmental 
duct), and that the anterior opening subsequently becomes divided 
up into the various apertures of the head-kidney. If this account 
is correct, Petromyzon presents a type intermediate between 
Amphibia and Elasmobranchii. In certain types, viz. Marsipo- 
branchii and Teleostei, the segmental duct becomes the duct for 
the posterior kidney (segmental tubes), but otherwise undergoes 
no further differentiation. Inthe majority of types, however, the 
case is different. In Amphibia,! as has already been mentioned, 
a solid rod of cells is split off from its ventral wall, which after- 
wards becomes hollow, acquires an opening into the body cavity, 
and forms the Miillerian duct. 

In Hlasmobranchii the segmental duct undergoes a more or 
less similar division. ‘It becomes longitudinally split into two 
complete ducts in the female, and one complete duct and parts of 
a second in the male. The resulting ducts are the (1) Wolffian 
duct dorsally, which remains continuous with the excretory 
tubules of the kidney, and ventrally (2) the oviduct or Millerian 
duct in the female, and the rudiments of this duct in the male. 
Tn the female the formation of these ducts takes place by a nearly 
solid rod of cells, being gradually split off from the ventral side of 
all but the foremost part of the original segmental duct, with 
the short undivided anterior part of which duct it is continuous 
in front. Into it a very small portion of the lumen of the original 
segmental duct is perhaps continued. The remainder of the 
segmental duct (after the loss of its anterior section and the part 
split off from its ventral side) forms the Wolffian duct. The 
process of formation of the ducts in the male chiefly differs from 
that in the female, in the fact of the anterior undivided part of 
the segmental duct, which forms the front end of the Miullerian 
duct, being shorter, and in the column of cells with which it is 
continuous being from the first incomplete.” 

It will be seen from the above that the Miillerian duct consists 
of two distinct parts—an anterior part with the abdominal open- 
ing, and a posterior part split off from the segmental duct. This 
double constitution of the Miillerian duct is of great importance 
for a proper understanding of what takes place in the Bird. 

The Miullerian duct appears, therefore, to develop in nearly the 
same manner in the Amphibian and Elasmobrauch type as a 
solid or nearly solid rod split off from the ventral wall of the 
segmental duct. But there is one important difference concern- 
ing the abdominal opening of the duct. In Amphibia this is a 
new formation, but in Hlasmobranchii it is the original open- 
ing of the segmental duct. Although we admit that in a large 
number of points, including the presence of a head-kidney, the 

1 Firbringer, loc. cit, 


16 BALFOUR AND SEDGWICK. 


urino-genital organs of Amphibia are formed on a lower type than 
those of the Elasmobranchi, yet it appears to us that this does 
not hold good for the development of the Miillerian duct. 

The above description will, we trust, be sufficient to render 
clear our views upon the development of the excretory system in 
Aves. 

In the bird the excretory system consists of the following parts 
(using the ordinary nomenclature) which are developed in the 
order below. 

1. Wolffian duct. 2. Wolffian body. 3. Head-kidney. 4. 
Miillerian duct. 5. Permanent kidney and ureter. 

About 2 and 5 we shall have nothing to say in the sequel. 

We have already in the early part of the paper given an account 
of the head-kidney and Miillerian duct, but it will be necessary for 
us to say a few words about the development of the Wolffian duct 
(so called). Without entering into the somewhat extended litera- 
ture on the subject, we may state that we consider that the recent 
paper of Dr. Gasser’ supplies us with the best extant account of 
the development of the Wolffian duct. 

The first trace of it which he finds is visible in an embryo with 
eight proto-vertebree as a slight projection from the intermediate 
cell mass towards the epiblast in the region of the three hinder- 
most proto-vertebre. In the next stage, with eleven proto-ver- 
tebree, the solid rudiment of the duct extends from the fifth to the 
eleventh proto-vertebra, from the eighth to the eleventh proto- 
vertebra it lies between the epiblast and mesoblast, and is quite 
distinct from both, and Dr. Gasser distinctly states that in its 
growth backwards from the eighth proto-vertebra the Wolffian 
duct never comes into continuity with the adjacent layers. 

In the region of the fifth proto-vertebra, where the duct was 
originally continuous with the mesoblast, it has now become free, 
but is still attached in the region of the sixth and to the eighth 
proto-vertebra. In an embryo with fourteen proto-vertebre 
the duct extends from the fourth to the fourteenth proto-vertebra, 
and is now free between epiblast and mesoblast for its whole 
extent. Itis still for the most part solid, though perhaps a small 
lumen is present in its middle part. Inthe succeeding stages the 
lumen of the duct gradually extends backwards and forwards, the 
duct itself also passes inwards till it acquires its final position 
close to the peritoneal epithelium; at the same time its hind 
end elongates till it comes into connection with the cloacal 
section of the hind-gut. It should be noted that the duct in its 
backward growth does not appear to come into continuity with 
the subjacent mesoblast, but behaves in this respect exactly as 
does the segmental duct in Elasmobranchii (vide note on p. 14), 

1 * Arch. fiir Mic. Anat.,’ vol. xiv. 


EXISTENCE OF HEAD-KIDNEY IN THE EMBRYO CHICK, 17 


The question which we propose to ourselves is the following :— 
What are the homologies of the parts of the Avian urinogenital 
system above enumerated? ‘The Wolffian duct appears to us mor- 
phologically to correspond 2m part to the segmental duct,! or 
what Fiirbringer would call the duct of the head-kidney. This 
may seem a paradox, since in birds it never comes into relation with 
the head-kidney. Nevertheless, we consider that this homology 
is morphologically established, for the following reasons :— 

(1) That the Wolffian duct gives rise (vide supra, p. 12) to the 
Millerian duct as well as to the duct of the Wolffian body. . In 
this respect it behaves precisely as does the segmental duct of 
Elasmobranchii and Amphibia. That it serves as the duct for 
the Wolffian body, before the Miillerian duct originates from it, 
is also in accordance with what takes place in other types. 

(2) That it develops in a strikingly similar manner to the 
segmental duct of Elasmobranchii. 

We stated expressly that the Wolffian duct corresponded only 
in part to the segmental duct. It does not, in fact, in our 
Opinion, correspond to the whole segmental duct, but to the 
segmental duct minus the anterior abdominal opening in 
Elasmobranchii, which becomes the head-kidney in other types. 
In fact, we suppose that the segmental duct and _head- 
kidney, which in the Ichthyopsida develop as a single formation, 
develop in the Bird as two distinct structures—one of these 
known as the Wolffian duct, and the other the head-kidney. 
If our view about the head-kidney is ‘accepted the above position 
will hardly require to be disputed, but we may point out that the 
only feature in which the Wolffian duct of the Bird differs in deve- 
lopment from the segmental duct of Hlasmobranchii is in the ab- 
sence of the knob, which forms the commencement of the segmental 
duct, and in which the abdominal opening is formed; so that 
the comparison of the development of the duct in the two types 
confirms the view arrived at from other considerations. 

The head-kidney and Miillerian duct in the Bird must be con- 
sidered together. The parts which they eventually give rise to 
after the atrophy of the head-kidney have almost universally been 
regarded as equivalent to the Millerian duct of the Ichthyopsida. 
By Braun,” however, who from his researches on the lizard satisfied 

1 The views here expressed about the Wolffian duct are nearly though not 
exactly those which one of us previously put forward (‘ Urinogenital Organs 
of Vertebrates,’ &c., p. 45-46), and with which Fiirbringer appears exactly 
to agree. Possibly Dr. Fiirbinger would alter his view on this point were he 
to accept the facts we believe ourselves to have discovered. Semper’s view 
also differs from ours, in that he believes the Wolffian duct to correspond 
in its entirety with the segmental duct. 


2 “Urogenital System d. Reptilien.” ‘Arb. aus d. zool.-zoot. Inst.’ 
Wirzburg, vol. iv. 
| 2 


18 BALFOUR AND SEDGWICK. 


himself of the entire independence of the Millerian and Wolffian 
ducts in the Amniota, the Miillerian duct of these forms is re- 
garded as a completely new structure with no genetic relations to 
the Miillerian duct of the Ichthyopsida. Semper’, on the other 
hand, though he accepts the homology of the Miillerian duct 
in the Ichthyopsida and Amniota, is of opinion that the anterior 
part of the Miillerian duct in the Amniota is really derived from 
the Wolffian duct, though he apparently admits the independent 
growth of the posterior part of the Miillerian duct. We have 
been led by our observations, as well as by our theoretical de- 
ductions, to adopt a view exactly the reverse of that of Professor 
Semper. We believe that the anterior part of the Miillerian duct 
of Aves, which is at first the head-kidney, and subsequently 
becomes the abdominal opening of the duet, is developed from 
the peritoneal epithelium independently of all other parts of the 
excretory system; but that the posterior part of the duct is more 
or less completely derived from the walls of the Wolffian duct. 
‘This view is clearly in accordance with our account of the facts 
of development in Aves, and it fits in very well with the develop- 
ment of the Miillerian duct in Hlasmobranchii. We have already 
pointed out that in Elasmobranchii the Millerian duct is formed 
of two factors—(1) of the whole anterior extremity of the seg- 
mental duct, including its abdominal opening; (2) of a rod split 
off from the ventral side of the segmental duct. In Birds the 
anterior part (corresponding to factor No. 1) of the Millerian 
duct has a different origin from the remainder; so that if the 
development of the posterior part of the duct (factor No. 2). 
were to proceed in the same manner in Birds and Elasmo- 
branchii, it ought to be formed at the expense of the Wolffian 
(z.e. segmental) duct, though in connection anteriorly with the 
head-kidney.- And this is what actually appears to take place. 

So far the homologies of the avian excretory system are fairly 
clear; but there are still some points which have to be dealt 
with in connection with the permanent opening of the Miillerian 
duct, and the relatively posterior position of the head-kidney. 
With reference to the first of these points the facts of the case 
are the following: 

In Amphibia the permanent opening of the Miillerian duct 
is formed as an independent opening after the atrophy of the 
head-kidney. 

In Elasmobranchii the original opening of the segmental duct 
forms the permanent opening of the Miillerian duct and no head- 
kidney appears to be formed. 

In Birds the anterior of the three openings of the head- 


1 Loc. cit. 


EXISTENCE OF HEAD-KIDNEY IN THE EMBRYO CHICK. 19 


kidney remains as the permanent opening of the Miillerian 
duct.. 

With reference to the difficulties involved in there being 
apparently three different modes in which the permanent opening 
of the Miillerian duct is formed, we would suggest the following 
considerations : i 

The history of the development of the excretory system-teaches 
us that primitively the segmental duct must have served as 
efferent duct both for the generative products and kidney secre- 
tion (just as the Wolffian duct still does for the testicular pro- 
ducts and secretion of the Wolffian body in Hlasmobranchii and 
Amphibia) ; and further, that at first the generative products 
entered the segmental duct from the abdominal cavity by one 
or more of the abdominal openings of the kidney (almost  cer- 
‘tainly of the head-kidney). That the generative products did 
not enter the segmental duct at first by an opening specially 
developed for them appears to us to follow from Dohrn’s 
principle of the transmutation of function (Functionswechsel). 
As a consequence (by a process of natural selection) of the seg- 
mental duct having both a generative and a urinary function, a 
further differentiation took place, by which that duct became split 
into two—a ventral Miillerian duct and dorsal Wolffian duct. 

The Millerian duct without doubt was continuous with the 
head-kidney, and so with the abdominal opening or openings of 
the head-kidney which served as generative pores. At first the 
segmental duct was probably split longitudinally into two equal 
portions, but the generative function of the Millerian duct gra- 
dually impressed itself more and more upon the embryonic deve- 
lopment, so that, in the course of time, the Miillerian duct 
developed less and less at the expense of the Wolffian duct. 
This process appears partly to have taken place in Elasmobranchii, 
and still more in Amphibia; the Amphibia offering in this 
respect a less primitive condition than Elasmobranchii; while in 
Aves it has been carried even further. The abdominal opening 
no doubt also became specialised. At first it is quite possible 
that more than one abdominal opening may have served for the 
generative products; one of which, no doubt, eventually came 
to function alone. In Amphibia the specialisation of the opening 
appears to have gone so far that it no longer has any relation to 
the head-kidney, and even develops after the atrophy of 
the head-kidney. In Elasmobranchii, on the other hand, the 
functional opening appears at a period when we should expect 
the head-kidney to develop. ‘This state is very possibly the 
result of a differentiation (along a different line to that in Am- 
phibia) by which the head-kidney gradually ceased to become 
developed, but by which the primitive opening (which in the 


20 BALFOUR AND SEDGWICK. 


development of the head-kidney used to be divided into several 
pores leading into the body cavity) remained undivided and 
served as the abdominal aperture of the Miillerian duct. Aves, 
finally, appear to have become differentiated along a third line ; 
since in their ancestors the anterior pore of the head-kidney 
appears to have become specialised as the permanent opening of 
the Miillerian duct. 

_ With reference to the posterior position of the head-kidney in. 
Aves we have only to remark, that a change in position of the 
head-kidney might easily take place after it acquired an inde- 
pendent development. The fact that it is slightly behind the 
glomerulus would seem to indicate, on the one hand, that it has 
already ceased to be of any functional importance ; and, on the 
other, that the shifting has been due to its having a connection 
with the Millerian duct. | 

We have made a few observations on the development of the 
Millerian duct in Lacerta muratis, which have unfortunately 
led us to no decided conclusions. In a fairly young stage in the 
development of the Miillerian duct (the youngest we have met 
with), no trace of a head-kidney could be observed, but the cha- 
racter of the abdominal opening of the Miillerian duct was very 
similar to that figured by Braun.’ As to the backward growth 
of the Millerian duct, we can only state that the solid point of 
the duct in the young stages is in contact with the wall of the 
Wolffian duct, and the relation between the two is rather like 
that figured by Firbringer (PI. I, figs. 14-15) in Amphibia. 


' Loe. cit. 


21 


On the Harty Devetorment of the Lacerti1ia, together 
with some OBsERVATIONS on the Nature and Reta- 
TIONS of the Primitive StreaK. By F. M. Batrovr, 
M.A., F.R.S., Fellow of Trinity College, Cam- 
bridge. (With Plate III.) 


Trxx quite recently no observations were recorded on the early 
developmental changes of the reptilian ovum. Not long ago 
Professors Kupffer and Benecke published a preliminary note 
on the early development of Lacerta agilis and Emys Europea.' 
I have myself also been able to make some observations on the 
embryo of Lacerta muralis. The number of my embryos has 
been somewhat limited, and most of those which I have had have 
been preserved in bichromate of potash, which has turned out a 
far from satisfactory hardening reagent. In spite of these diffi- 
culties I have been led on some points to very different results 
from those of the German investigators, and to results which are 
more in accordance with what we know of other Sauropsidan 
types. J commence with a short account of the results of 
Kupffer and Benecke. 

Segmentation takes place exactly as in birds, and the resulting 
blastoderm, which is thickened at its edge, spreads rapidly over 
the yolk. Shortly before the yolk is half enclosed a small 
embryonic shield (area pellucida) makes its appearance in the 
centre of the blastoderm, which has, in the meantime, become 
divided into two layers. The upper of these is the epiblast, and 
the lower the hypoblast. The embryonic shield is mainly distin- 
guished from the remainder of the blastoderm bythe more columnar 
character of its constituent epiblast cells. It is somewhat pyri- 
form in shape, the narrower end corresponding with the future 
posterior end of the embryo. At the narrow end an invagina- 
tion takes place, which gives rise to an open sac, the blind end of 
which is directed forwards. The opening of this sac is regarded 
by the authors as the blastopore. A linear thickening of epi- 
blast arises in front of the blastopore, along the median line of 
which the medullary groove soon appears. In the caudal region 
the medullary folds spread out and enclose between them the 
blastopore, behind which they soon meet again. On the con- 
version of the medullary groove into a closed canal the blastopore 
becomes obliterated. The mesoblast grows out from the lip of 
the blastopore as four masses. Two of these are lateral: a third 


M4 Die Erste Entwicklungsvorginge am Ei der Reptilien,’ Konigsberg, 


+ a Fr, M. BALFOUR. 


is anterior and median, and, although at first independent of the 
epiblast, soon attaches itself to it, and forms with it a kind of 
axis-cord. A fourth mass applied itself to the walls of the sac 
formed by invagination. | 

With reference to the very first develogmental phenomena my 
observations are confined to two stages during the segmentation. 
In the earliest of these the segmentation was about half completed, 
in the later one it was nearly over. My observations on these 
stages bear out generally the statements of Kupffer and Benecke. 
In the second of them the blastoderm was already imperfectly 
divided into two layers—a superficial epiblastic layer formed of a 
single row of cells, and a layer below this several rows deep. 
Below this layer fresh segments were obviously being added to 
the blastoderm frem the subjacent yolk. 

Between the second of these blastoderms and my next stage 
there is a considerable gap. The medullary plate is just 
established, and is marked by a shallow groove which becomes 
deeper in front. A section through the embryo is represented 
in Pl. III, Series a, fig. 1. In this figure there may be 
seen the thickened medullary plate with a shallow medullary 
groove, below which are two independent plates of mesoblast 
(me. p.), one on each side of the middle line, very imperfectly 
divided into somatopleuric and splanchnopleuric layers. Below 
the mesoblast is a continuous layer of hypoblast (4y.), which 
develops a rod-like thickening along the axial line (cd.). This 
rod becomes in the next stage the notochord. Although this 
embryo is not well preserved 1 feel very confident in asserting the 
continuity of the notochord with the hypoblast at this stage. 

At the hind end of the embryo is placed a thickened ridge of 
tissue which continues the embryonic axis. In this ridge all the 

“layers coalesce, and I therefore take it to be equivalent to the 
primitive streak of the avian blastoderm. It is somewhat 
triangular in shape, with the apex directed backward, the broad 
base placed in front. 

At the junction between the primitive streak and the blasto- 
derm is situated a passage, open at both extremities, leading 
from the upper surface of the blastoderm obliquely forwards to 
the lower. 

The dorsal and anterior wall of this passage is formed of a 
distinct epithelial layer, continuous at its upper extremity with 
the epiblast, and at its lower with the notochordal plate, so that 
it forms a layer of cells connecting together the epiblast and hypo- 
blast. The hinder and lower wall of the passage is formed by the 
cells of the primitive streak, which only assume a columnar form 


1 For these two specimens, which were hardened in picric acid, I am 
indebted to Dr. Kleneinberg. 


EARLY DEVELOPMENT OF THE LACERTILAA. 23 


near the dorsal opening of the passage (vide fig. 4). This passage 
is clearly the blind sac of Kupffer and Benecke, who, if I am not 
mistaken, have overlooked its lower opening. As I hope to show 
in the sequel, it is also the equivalent of the neurenteric passage, 
which connects the neural and alimentary canals in the Icthyop- 
sida, and therefore represents the blastopore of Amphioxus 
Amphibians, &c. 

Series A, figs. 2, 8, 4, 5, illustrate the features of the passage 
and its relation to the embryo. 

Fig. 2 passes through the ventral opening of the passage. 
The notochordal plate (ci’.) is vaulted over the opening, and 
on the left side is continuous with the mesoblast as well as the 
hypoblast. Figs 3 and 4 are taken through the middle part of 
the passage (ve.), which is bounded above by a continuation of 
the notochordal plate, and below by the tissue of the primitive 
streak. The hypoblast (Ay.), in the middle line, is imperfectly 
fused with the mesoblast of the primitive streak, which is now 
continuous across the middle line. The medullary groove has 
disappeared, but the medullary plate (m p.) is quite distinct. 

In fig. 5 is seen the dorsal opening of the passage (we.). If 
a section behind this had been figured, as is done for the next 
series (B), it would have passed through the primitive streak, 
and, as in the chick, all the layers would have been fused 
together. The epiblast in the primitive streak completely 
coalesces with the mesoblast; but the hypoblast, though attached 
to the other layers in the middle line, can always be traced as a 
distinct stratum. 

Fig. B is a surface view of my next oldest embryo. The 
medullary groove has become much deeper, especially in front. 
Behind it widens out to form a space equivalent to the sinus 
rhomboidalis of the embryo bird. The amnion forms a small fold 
covering over the cephalic extremity of the embryo, which is 
deeply embedded in the yolk. Some somites (protovertebre) 
were probably present, but this could not be made out in the 
opaque embryo. 

The woodcut (fig. 1) represents a diagrammatic longitudinal 


Fic. 1.-Diagrammatic longitudinal section of an embryo of Lacerta. 
pp. Body cavity. am. Amnion. ne. Neurenteric canal. ch. 
ono hy. Uypoblast. ep. Bpiblast. pr. Primitive 
streak, 


24 F, M. BALFOUR. 


section through this embryo, and the sections belonging to Series 
B illustrate the features of the hind end of the embryo and of the 
primitive streak. 

As is shown in fig. 1, the notochord (ch.) has now throughout 
the region of the embryo become separated from the subjacent 
hypoblast, and the lateral plates of mesoblast are distinctly 
divided into somatic and splanchnic layers. The medullary groove 
is continued as a deepish groove up to the opening of the neuren- 
teric passage, which thus forms a perforation in the floor of the 
hinder end of the medullary groove (vide Series 8, figs. 2, 3, 
and 4). 

The passage itself is somewhat shorter than in the previous 
stage, and the whole of it is shown in a single section (fig. 4). 
This section must either have been taken somewhat obliquely, or 
else the passage have been exceptionally short in this embryo, 
since in an older embryo it could not all be seen in one 
section. 

The front wall of the passage is continuous with the notochord, 
which for two sections or so in front remains attached to the hypo- 
blast (figs. 2 and 3). Behind the perforation in the floor of the 
medullary groove is placed the primitive streak (fig. 5), where all 
the layers become fused together, as in the earlier stage. Into 
this part a narrow diverticulum from the end of the medullary 
groove is continued for a very short distance (wide fig. 5, me.). 

The general features of the stage will best be understood by an 
examination of the diagrammatic longitudinal section, represented 
in woodcut, fig. 1. In front is shown the amnion (am.), growing 
over the head of the embryo. The notochord (cd.) is seen as an 
independent cord for the greater part of the length of the embryo, 
but falls into the hypoblast shortly in front of the neurenteric 
passage. The neurenteric passage is shown at we., and behind it 
is shown the primitive streak. 

In a still older stage, represented in surface view on PI. III, 
fig. c, medullary folds have nearly met above, but have not yet 
united. The features of the passage from the neural groove to the 
hypoblast are precisely the same in the embryo just described, 
although the lumen of the passage has become somewhat narrower. 
There is still a short primitive streak behind the embryo. 

The neurenteric passage persists but a very short time after 
the complete closure of the medullary canal. It is in no way 
connected with the allantois, as conjectured by Kupffer and 
Benecke, but the allantois is formed, as I have satisfied myself. 
by longitudinal sections of a later stage, in the manner already 
described by Dobrynin, Gasser, and Kolliker for the bird and 
mammal. 

The general results of Kupffer’s and Benecke’s observations, 


EARLY DEVELOPMENT OF THE LACERTILIA. 25 


with the modifications introduced by my own observations, are 
as follows :—After the segmentation and the formation of the 
embryonic shield (area pellucida) the blastoderm becomes dis- 
tinctly divided into epiblast and hypoblast.!. At the hind end of 
the shield a somewhat triangular primitive streak is formed by 
the fusion of the epiblast and hypoblast with a number of cells 
between them, which are probably derived from the lower rows 
of the segmentation cells. At the front end of the streak a 
passage arises, open at both extremities, leading obliquely 
forwards through the epiblast to the space below the hypo- 
blast. The walls of the passage are formed of a layer of 
columnar cells continuous both with epiblast and hypoblast. 
In front of the primitive streak the body of the embryo 
becomes first differentiated by the formation of a medullary 
plate, and at the same time there grows out from the primitive 
streak a layer of mesoblast, which spreads out in all directions 
between the epiblast and hypoblast. In the axis of the embryo 
the mesoblast plate is stated by Kupffer and Benecke to be con- 
tinuous across the middle line, but this appears very improbable. 
In a slightly later stage the medullary plate becomes marked by 
a shallow groove, and the mesoblast of the embryo is then un- 
doubtedly constituted of two lateral plates, one on each side of 
the median line. In the median line the notochord arises as a 
ridge-like thickening of the hypoblast which becomes very soon 
quite separated from the hypoblast, except at the hind end, 
where it is continued into the front wall of the neurenteric pas- 
sage. It is interesting to notice the remarkable relation of the 
notochord to the walls of the neurenteric passage. More or less 
similar relations are also well marked in the case of the goose and 
the fowl (Gasser),? and support the conclusion deducible from 
the lower forms of vertebrata, that the notochord is essentially 
hypoblastic. 

‘The passage at the front end of the primitive streak forms the 
posterior boundary of the medullary plate, though the medullary 
groove is not at first continued back to it. The anterior wall of 
this passage connects together the medullary plate and the noto- 
chordal ridge of the hypoblast. In the succeeding stages the 
medullary groove becomes continued back to the opening of the 
passage, which then becomes enclosed in the medullary folds, 
and forms a true neurenteric passage. It becomes narrowed as 
the medullary folds finally unite to form the medullary canal, 
and eventually disappears. 

I conclude this paper with a concise statement of what 

1 This appears to me to take place before the formation of the em- 


bryonic shield. 
2 Gasser, ‘ Der Primitivstreifen bei Vogelembryonen,’ Marburg, 1878. 


26 F. M. BALFOUR. 


appears to me the probable nature of the much-disputed organ, 
the primitive streak, and of the arguments in support of my 
view. 

In a paper on the primitive streak in the ‘Quart. Journ. of 
Mic. Sci.,’ in 1873 (p. 280), I made the following statement with 
reference to this subject:—‘‘ It is clear, therefore, that the 
primitive groove must be the rudiment of some ancestral feature. 
piweieia.. It is just possible that it is the last trace of that 
involution of the epiblast by which the hypoblast is formed in 
most of the lower animals.” 

At a later period, in July, 1876, after studying the develop- 
ment of Hlasmobranch fishes, I enlarged the hypothesis in a re- 
view of the first part of Prof. Kolliker’s ‘ Entwicklungsgeschichte.’ 
The following is the passage in which I speak of it :} 

“Tn treating of the exact relation of the primitive groove to the 
formation of the embryo, Professor Kélliker gives it as his view 
that though the head of the embryo is formed independently 
of the primitive groove, and only secondarily unites with this, 
yet that the remainder of the body is without doubt derived 
from the primitive groove. With this conclusion we cannot 
agree, and the very descriptions of Professor Kolliker appear to 
us to demonstrate the untenable nature of his results. We 
believe that the front end of the primitive groove at first occu- 
pies the position eventually filled by about the third pair of 
protovertebre, but that as the protovertebre are successively 
formed, and the body of the embryo grows in length, the primi- 
tive groove is carried further and further back, so as always to 
be situated immediately behind the embryo. As Professor K@l- 
liker himself has shown it may still be seen in this position even 
later than the fortieth hour of incubation. 

“Throughout the whole period of its existence it retains a 
character which at once distinguishes it in sections from the 
medullary groove. 

“ Beneath it the epiblast and mesoblast are always fused, 
though they are always separate elsewhere; this fact, which was 
originally shown by ourselves, has been very clearly brought out 
by Professor Kolliker’s observations. 

“The features of the primitive groove which throw special 
light on its meaning are the following : 

“(1.) It does not enter directly into the formation of the 
embryo. 

“(2.) The epiblast and mesoblast always become fused 
beneath it. 

“« (3.) It is situated immediately behind the embryo. 


‘ * Journal of Anat. and Phys.,’ vol. x, pp. 790 and 791. Compare als 
my monograph on ‘ Elasmobranch Fishes,’ note on p. 68. ; 


“BARLY DEVELUPMENT OF THE LACERTILIA. 27 


“ Professor Kélliker does not enter into any speculations as to 
the meaning of the primitive groove, but the above-mentioned 
facts appear to us clearly to prove that the primitive groove is a 
rudimentary structure, the origin of which can only be com- 
pletely elucidated by a knowledge of the development of the 
Avian ancestors. 

“Tn comparing the blastoderm of a bird with that of any 
apamniotic vertebrate, we are met at the threshold of our inves- 
tigations by a remarkable difference between the two. Whereas 
in all the lower vertebrates the embryo is situated at the edge 
of the blastoderm, it is in birds and mammals situated in the 
centre. ‘This difference of position at once suggests the view 
that the primitive groove may be in some way connected with the 
change of position in the blastoderm which the ancestors of 
birds must have undergone. If we carry our investigations 
amongst the lower vertebrates a little further, we find that the 
Elasmobranch embryo occupies at first the normal position at 
the edge of the blastoderm, but that in the course of develop- 
ment the blastoderm grows round the yolk far more slowly in 
the region of the embryo than elsewhere. Owing to this, the 
embryo becomes left in a bay, the two sides of which eventually 
meet and coalesce in a linear fashion immediately behind the em- 
bryo, thus removing the embryo from the edge of the blasto- 
derm and forming behind it a linear streak not unlike the primi- 
tive streak. We would suggest the hypothesis that the primitive 
groove is a rudiment which gives the last indication of a change 
made by the Avian ancestors in their position in the blastoderm, 
like that made by Elasmobranch embryos when removed from the 
edge of the blastoderm and placed ina central situation similar 
to that of the embryo bird. On this hypothesis the situation of 
the primitive groove immediately behind the embryo, as well as 
the fact of its not becoming converted into any embryonic organ 
would be explained. The central groove might probably also 
be viewed as the groove naturally left between the coalescing 
edges of the blastoderm. 

“‘ Would the fusion of epiblast and mesoblast also receive its ex- 
planation on this hypothesis? We are of opinion that it would. At 
the edge of the blastoderm which represents the blastopore mouth 
of Amphioxus all the layers become fused together in the anam- 
niotic vertebrates. So that if the primitive groove is in reality 
a rudiment of the coalesced edges of the blastoderm, we might 
naturally expect the layers to be fused there, and the difficulty 
presented by the present condition of the primitive groove would 
rather be that the hypoblast is not fused with the other layers 
than that the mesoblast is indissolubly united with the epi- 
blast, The fact that the hypoblast is not fused with the other 


28 F, M. BALFOUR, 


layers does not appear to us to be fatal to our hypothesis, and in 
Mammalia, where the primitive and medullary grooves present pre- 
cisely the same relations as in birds, all three layers are, accord- 
ing to Hensen’s account, fused: together. This, however, is 
denied by Kélliker, who states that in Mammals, as in Birds, only 
the epiblast and mesoblast fuse together. Our hypothesis as to 
the origin of the primitive groove appears to explain in a fairly 
satisfactory manner all the peculiarities of this very enigmatical 
organ; it also relieves us from the necessity of accepting Professor 
Kolliker’s explanation of the development of the mesoblast, though 
it does not, of course, render that explanation in any way 
untenable.” 

At a somewhat later period Rauber arrived at a more or less 
similar conclusion, which, however, he mixes up with a number 
of opinions from which I am compelled altogether to dissent.1 

The general correctness of my view, as explained in my second 
quotation, appears to me completely established by Gasser’s beau- 
tiful researches on the early development of the chick and goose, 
and by my own observations just recorded on the lizard. 
While at the same time the parallel between the blastopore 
of Hlasmobranchii and of the Sauropsida, is rendered more com- 
plete by the discovery of the neurenteric passage in the latter 
group, which was first of all made by Gasser. 

The following paragraphs contain a detailed attempt to establish 
the above view by a careful comparison of the primitive streak 
and its adjuncts in the amniotic vertebrates with the blastopore 
in Hlasmobranchi.. 

In Hlasmobranchii the blastopore consists of the following 
parts :—(1), a section at the end of the medullary plate, which 
becomes converted into the neurenteric canal ;? (2), a section 
forming what may be called the yolk blastopore, which even- 
tually constitutes a linear streak connecting the embryo with the 
edge of the blastoderm (#7de my monograph on Elasmobranch 
fishes, pp. 68 and 81). In order to establish my hypothesis 
on the nature of the primitive streak, it is necessary to find the 
representatives of both these parts in the primitive streak of the 
amniotic vertebrates. ‘The first section ought to appear as a 
passage from the neural to the enteric side of the blastoderm 
at the posterior end of the medullary plate. At its front 
edge the epiblast and hypoblast should be continuous, as they 
are at the hind end of the embryo in Elasmobranchii, and, 


1 “ Primitivrinne u. Urmuxd,” ‘ Morphologisches Jahrbuch.,’ Band ii, p. 
551. 

2 Gasser, ‘Der Primitivstreifen bei Vogelembryonen,’ Marburg, 1878. 

3 I use this term for the canal connecting the neural and alimentary 
tract, which was first discovered by Kowalevsky. : 


EARLY DEVELOPMENT OF THE LACERTILIA. 29 


finally, the passage should, on the closure of the medullary 
groove, become converted into the meurenteric canal. All these 
conditions are exactly fultilled by the opening at the front end of 
the primitive streak of the lizard (wide woodcut, fig. 1). In 
the chick there is at first no such opening, but, as I hope to 
show in a future paper, it is replaced by the epiblast and hypo- 
blast falling into one another at the front end of the primitive 
streak. At a later period, as has been shown by Gasser,! there 
is a distinct rudiment of the neurenteric canal in the chick, and a 
complete canal in the goose. Finally, in mammals, as has been 
shown by Schafer? for the guinea-pig, there is at the front end 
of the primitive streak a complete continuity between epiblast 
and hypoblast. The continuity of the epiblast and hypoblast at 
the hind end of the embryo in the bird and the mammal is 
a rudiment of the continuity of these layers at the dorsal lip of 
the blastopore in Elasmobranchii, Amphibia, &c. The second 
section of the blastopore in Elasmobranchii or yolk blastopore is, 
I believe, partly represented by the primitive streak. The yolk 
blastopore in Elasmobranchii is the part of the blastopore belong- 
ing to the yolk sac as opposed to that belonging to the embryo, 
and it is clear that the primitive streak cannot correspond to the 
whole of this, since the primitive streak is far removed from the 
edge of the blastoderm long before the yolk is completely enclosed. 
Leaving this out of consideration the primitive streak, in order 
that the above comparison may hold good, should satisfy the 
following conditions : 

1. It should connect the embryo with the edge of the blasto- 
derm. 

2. It should be constituted as if formed of the fused edges of 
the blastoderm. 

3. The epiblast of it should eventually not form part of the 
medullary plate of the embryo, but be folded over on to the 
ventral side. | 

The first of these conditions is only partially fulfilled, but, con- 
sidering the rudimentary condition of the whole structure, no 
great stress can, it seems to me, be laid on this fact. 

The second condition seems to me very completely satisfied. 
Where the two edges of the blastoderm become united we should 
expect to find a complete fusion of the layers such as takes place 
in the primitive streak ; and the fact that in the primitive streak 
the hypoblast does not so distinctly coalesce with the mesoblast 
as the mesoblast with the epiblast cannot be urged as a serious 
argument against me. 

1 Loe. cit. 


2 “ A contribution to the history of the development in the Guinea-pig,” 
‘Journal of Anat. and Phys.,’ vol. xi, pp. 332—336. 


80 ¥. M. BALFOUR. 


The growth outwards of the mesoblast from the axis of the 
primitive streak is probably a remnant of the invagination of the 
hypoblast and mesoblast from the lip of the blastopore in 
Amphibia, &c. | 

The groove in the primitive streak may with great plausibility 
be regarded as the indication of a depression which would natu- 
rally be found along the line where the thickened edges of the 
blastoderm became united. 

With reference to the third condition, I will make the following 
observations. The neurenteric canal, as it is placed at the extreme 
end of the embryo, must necessarily, with reference to the embryo, 
be the hindermost section of the blastopore, and therefore the 
part of the blastopore apparently behind this can only be so owing’ 
to the embryo not being folded off from the yolk sac; andas the 
yolk sac is in reality a specialised part of the ventral wall of the 
body, the yolk blastopore must also be situated on the ventral 
side of the embryo. 

Kolliker and other distinguished embryologists have believed 
that the epiblast of the whole of the primitive streak became part 
of the neural plate. If this view were correct, which is accepted 
even by Rauber, the hypothesis [am attempting to establish would 
fall to the ground. I have, however, no doubt that these em- 
bryologists are mistaken. The very careful observations of 
Gasser show that the part of the primitive streak adjoining the 
embryo becomes converted into the tail-swelling, and that the 
posterior part is folded in on the ventral side of the embryo, and, 
losing its characteristic structure, forms part of the ventral wall 
of the body. On this point my own observations confirm those 
of Gasser. In the lizard the early appearance of the neurenteric 
canal at the front end of the primitive streak clearly shows that 
here also the primitive streak can take no share in forming the 
neural plate. 

The above considerations appear to me sufficient to establish 
my hypothesis with reference to the nature of the primitive 
streak, which has the merit of explaining, not only the structural 
peculiarities of the primitive streak, but also the otherwise inex- 
plicable position of the embryo of the amniotic vertehrates in 
the centre of the blastoderm. 


31 


On Certain Ports in the Anatomy of. PERIPATUS 
Carensis. By F. M. Batrour, M.A., F.R.S.* 


TuE discovery by Mr. Moseley? of a tracheal system in Peri- 
patus must be reckoned as one of the most interesting results 
obtained by the naturalists of the “Challenger.” The discovery 
clearly proves that the genus Peripatus, which is widely dis- 
tributed over the globe, is the persisting remnant of what was 
probably a large group of forms, from which the present tracheate 
Arthropoda are descended. 

The affinities of Peripatus render any further light on its 
anatomy a matter of some interest; and through the kindness of 
Mr. Moseley I have had an opportunity of making investigations 
on some well-preserved examples of Peripatus capensis, a few of 
the results of which I propose to lay before the Society. 

T shall confine my observations to three organs. (1) The seg- . 
mental organs, (2) the nervous system, (3) the so-called fat 
bodies of Mr. Moseley. 

In all the segments of the body, with the exception of the first 
two or three postoral ones, there are present glandular bodies, 
apparently equivalent to the segmental organs of Annelids. 

These organs have not completely escaped the attention of pre- 
vious observers. The anterior of them were noticed by Grube,° 
but their relations were not made out. By Saenger,* as I gather 
from Leuckart’s ‘ Bericht’ for the years 1868-9, these structures 
were also noticed, and they were interpreted as segmental organs. 
Their external openings were correctly identified. They are not 
mentioned by Moseley, and no notice of them is to be found in the 
text-books. The observations of Grube and Saenger seem, in 
fact, to have been completely forgotten. 

The organs are placed at the bases of the feet in two lateral 
divisions of the body-cavity shut off from the main central median 
division of the body-cavity by longitudinal septa of transverse 
muscles. 

Each fully developed organ consists of three parts : 

) A dilated vesicle opening externally at the base of 
a foot. 


1 From the ‘ Proceedings of the Cambridge Philosophical Society.’ 

2 “On the Structure and Development of Peripatus Capensis,” ‘Phil. 
Trans.,’ vol. clxiv, 1874. 

3 “Bau von Perip. Hdwardsti,” ‘ Archiv f. Anat. u. Phys.,’ 1853. 

4 “ Moskauer Naturforscher Sammlung,” ‘ Abth. Zool.,’ 1869. 


32 F, M. BALFOUR. 


(2) A coiled glandular tube connected with this and subdi- 
vided again into several minor divisions. 

(3) A short terminal portion opening at one extremity into the 
coiled tube (2) and at the other, as I believe, into the body-cavity. 
This section becomes very conspicuous in stained preparations by 
the intensity with which the nuclei of its walls absorb the colour- 
ing matter. , 

The segmental organs of Peripatus, though formed on a type of 
their own, more nearly resemble those of the Leech than of any 
other form with which I am acquainted. The annelidan affinities 
shown by their presence are of some interest. Around the seg- 
mental organs in the feet are peculiar cells richly supplied with 
tracheze, which appear to me to be similar to the fat bodies in 
insects. There are two glandular bodies in the feet in addition to 
the segmental organs. 

The more obvious features of the nervous system have been 
fully made out by previous observers, who have shown that it 
consists of large-paired supracesophageal ganglia connected with 
two widely separated ventral cords—stated by them not to be 
ganglionated. Grube describes the two cords as falling into one 
another behind the anus—a feature the presence of which is 
erroneously denied by Saenger. The lateral cords are united 
by numerous (5 or 6 for each segment) transverse cords. 

The nervous system would appear at first sight to be very 
lowly organised, but the new points I believe myself to have made 
out, as well as certain previously known features in it, appear to 
me to show that this is not the case. 

The following is a summary of the fresh points I have observed 
in the nervous system : 

(1) Immediately underneath the csophagus the cesophageal 
commissures dilate and forma pair of ganglia equivalent to the 
annelidan and arthropodan subeesophageal ganglia. These ganglia 
are closely approximated and united by 5 or 6 commissures. They 
give off large nerves to the oral papille. 

(2) The ventral nerve cords are covered on their ventral side by 
a thick ganglionic layer,! and at each pair of feet they dilate into a 
small but distinct ganglionic swelling. From each ganglionic 
swelling are given off a pair of large nerves” to the feet; and the 
ganglionic swellings of the two cords are connected together by 
a pair of commissures containing ganglion cells. The other com- 


1 This was known to Grube, loc. cit. 

* These nerves were noticed by Milne Edwards, but Grube failed to 
observe that they were much larger than the nerves given off between the 
feet. 

3 These commissures were perhaps observed by Saenger (loc. cit.). 


POINTS IN THE ANATOMY OF PERIPATUS CAPENSIS., 33 


missures connecting the two cords together do not contain ganglion 
cells. 

The chief feature in which Peripatus was supposed to differ 
from normal Arthropoda and Annelida, viz. the absence of ganglia 
on the ventral cords, does not really exist. In other par- 
ticulars, as in the amount of nerve cells in the ventral cords and 
the completeness of the commissural connections between the two 
cords, &c., the organisation of the nervous system of Peripatus 
ranks distinctly high. The nervous system lies within the circu- 
lar and longitudinal muscles, and is thus not in proximity with the 
skin. In this respect also Peripatus shows no signs of a primi- 
tive condition of the nervous system. 

A median nerve is given off from the posterior border of the 
supracesophageal ganglion to the cesophagus, which probably 
forms a rudimentary sympathetic system. I believe also that I 
have found traces of a paired sympathetic system. 

The organ doubtfully spoken of by Mr. Moseley as a fat body, 
and by Grube as a lateral canal, is in reality a glandular tube, 
lined by beautiful columnar cells containing secretion globules, 
which opens by means of a non-glandular duct intothe mouth. It 
lies close above the ventral nerve cords in a lateral compartment 
of the body-cavity, and extends backwards for a varying distance. 

This organ may perhaps be best compared with the simple 
salivary gland of Julus. It is not to be confused with the slime 
glands of Mr. Moseley, which have their opening in the oral 
papille. IfI am correct in regarding it as homologous with 
the salivary glands so widely distributed amongst the Tracheata, 
its presence indicates a hitherto unnoticed arthropodan affinity in 
Peripatus. , 


On some Points in the Harty Devetopment of the 
Common Newt. By W. B. Scort, B.A., Fellow 
of the College of New Jersey, Princeton, and 
Henry F. Osporn, B.A., Princeton. With 
Plates IV and V. 


THE present paper records a series of observations on 
the development of Triton teniatus (and partially also T. 
cristatus), made by the writers in the Morphological 
Laboratory of the University of Cambridge. 

It deals chiefly: (1) with the formation and character of 
the germinal layers; (2) the development of the notochord ; 
(3) the extension of the body cavity into the head, and the 
formation of mesoblastic somites in that region; (4) the 
development of the thyroid body. 

With a view of making the following account as clear as 
possible, we have chosen a series of embryos showing the 
most important steps in development, and have designated 
the stages which they represent by letters in imitation of the 
plan adopted by Mr. Balfour in his ‘Monograph on the 
Development of the Elasmobranch Fishes.’ And we have 
further endeavoured to make these stages correspond to those 
of Bombinator igneus, as figured by Dr. Gotte! in his great 
work. As might be expected, Triton in many ways shows a 
close resemblance to the Batrachian,*? and yet at the same 
time it presents a number of curious and striking differences 
from that type. In order to elucidate these we have followed 
Dr. Gotte’s arrangement as far as practicable. 

The preparation of the Triton embryos was attended with 
considerable difficulty. It was found in all cases advisable 
to remove the albumen from the ovum before hardening. 
The vitellus is quite liquid, and the vitelline membrane is so 
excessively delicate that this operation must be conducted 
with the greatest care; and as the albumen is permeated by 
several membranes, it was found necessary to cut these 
with fine scissors before the embryo could be with safety 


1 A. Gotte, ‘ Entwickelungsgeschicte der Unke.’ 
2 The term “ Batrachia,” is used in this paper in the restricted sense as 
equivalent to the Anurous Amphibia, 


EARLY DEVELOPMENT OF THE COMMON NEWT, 35 


extracted. Many hardening reagents were experimented 
with—osmic acid, bichromate of potash, Miller’s fluid, &c., 
but the most satisfactory one proved to be Kleinenberg’s 
pivric acid, with which nearly all the embryos described in 
the following pages were prepared. In those cases where the 
entire egg was hardened without previously removing the 
albumen, the results were most unsatisfactory. Kleinen- 
berg’s heematoxylin was the staining fluid employed for the 
sections. 


A. 


This includes embryos intermediate in age between 
Gotte’s figs. 39 and 40, taf. ii. The blastospore is quite 
small, a narrow groove, the “ Riickenrinne,” running forward 
some distance from its anterior edge. The medullary folds 
do not as yet appear in surface views. The ovum is still 
almost perfectly spherical in shape. 


B (Unke, Taf. i, figs. 40 and 41). 


At this stage the medullary folds become well developed 
and very plainly marked. As yet they are widely separated. 
The medullary plate is formed, but the groove which divides 
it into two parts does not reach far forwards of the middle ; 
or, at any rate, if present anteriorly, isextremely faint. The 
ovum has elongated very slightly, but still appreciably. 


C (Taf. iii, fig. 42). 


The medullary folds now become still more pronounced, 
and begin to approach each other. The point of closest 
approximation is in the region which will eventually become 
the neck, and here is the first point of contact, just as it is 
in the Batrachia. The medullary plate is plainly divided 
throughout. The elongation of the embryo is not much 
more marked than it was in the previous stage. 


D (see Pl. V, fig. 16). 


Up to this stage no important external differences between 
‘Triton and Bombinator are apparent, but now a number of 
points of divergence begin to be noticeable. The medullary 
folds have closed throughout the region of the trunk, but 
still remain open in the head. Posteriorly they separate to 
form a sinus rhomboidalis ; this does not seem to be merely a 
part of the canal which has not yet closed, but a genuine 
dilatation. It is either absent or very transitory in Bombt- 


36 W. B. SCOTT AND HENRY F, OSBORN. 


nator. As the folds enclose the blastopore, which remains 
open till a much later period, the sinus gives a communica- 
tion from the exterior to the alimentary canal. When the 
sinus. closes there is still the communication between the 
neural and alimentary canals, which has now been observed 
in so many types (Amphioxus, Accipenser, Elasmobranchii, 
Bombinator, &c.). The elongation of the embryo becomes 
very decided, and one surface of it becomes nearly flat; in 
Bombinator this is the dorsal surface; in the Newt it is the 
ventral, so that the latter is curved over the yolk. This 
difference is due merely to the larger amount of food-yolk 
in the egg of the Urodele, and cannot be considered of any 
great morphological significance. The bearings of the 
increased quantity of food-yolk will be discussed further on. 


E. 


This stage includes embryos, perhaps not quite so far 
advanced as the one figured in Gotte’s Taf. ini, fig. 50. The 
closure of the medullary folds is now complete throughout, 
and the vesicles of the brain are obscurely marked. The 
cranial flexure is already decided, and the whole embryo is 
somewhat curved upon itself, causing the ventral surface 
to assume a concave outline (except posteriorly, where the large 
mass of yolk produces a bulge). A trace of the opening of 
the sinus is still apparent. 


F (Taf. iui, fig. 52). 

The ventral curvature now becomes stronger, as does alsothe 
cranial flexure. The curvature is in an opposite direction to 
that taken by Bombinator. The vesicles of the brain are very 
distinct, and the optic vesicles which commenced in the last 
stage are now remarkably large, much more conspicuous than 
in the Bombinator of corresponding age. Another difference 
presents itself in the fact that in the latter the optic vesicle 
is an elongated oval, while in the former it is hemispherical. 
The rudiments of the fifth and seventh pairs of cranial 
nerves appear as buds from near the dorsal part of the hind 
brain, higher up than in Bombinator. A few protovertebre 
have been formed. Up to this time there has been little or 
no increase in absolute size, the changes in form being pro- 
duced by the elongation and narrowing of the embryo. 


G. 


In this stage the cranial flexure is carried further, and the 
head, as a whole, has taken a spherical shape, very different 
from the shape assumed by the Batrachian head. The 


EARLY DEVELOPMENT OF THE COMMON NEWT. 37 


rudiments of the visceral arches appear, and the tail begins 
tv bud out from the yolk sac as an unsegmented mass of 
mesoblast. The number of somites has increased. 


H (Taf. iii, fig. 53). 


The elongation of the embryo has now progressed to a 
very considerable extent. The cerebral hemispheres bud out 
as an unpaired rudiment from the forebrain. Four visceral 
arches and three clefts have been formed. The tail has elon- 
gated somewhat, and is still unsegmented. We have been 
unable to discover anything of the suckers or horny teeth 
found in the Batrachian larve. 


I (Taf. iii, fig. 54). (See also Pl. V, fig. 17). 


This stage exhibits a general advance in development, but 
the only new feature is the appearance of the involution for 
the mouth. This is transversely elongated, differing from 
the mouth involution of Bombinator. The head shows 
swellings, which correspond in position to those which Gotte 
has named, respectively, kidney swelling, lateral nerve, 
seventh and fifth nerves, auditory vesicle, and Gasserian gan- 
glion; but, owing to the fact that the curvature is in the 
opposite direction, these organs are separated by wider inter- 
vals than in Bombinator. 

We shall have occasion to refer to one or two later stages 
(K and L), which are marked by general increase in size, 
the formation of the lens, and the appearance of the external 
gills. 


Segmentation and Formation of the Layers. 


We have not succeeded in securing a complete series of 
specimens showing all the stages of segmentation, but from 
those which we have observed there can be little doubt that 
it proceeds very much in the same manner as in the Frog. 
Segmentation is asymmetrical, and this characteristic begins 
to appear at a very early period. The earliest stage we have 
seen shows two longitudinal furrows, which cut each other at 
right angles at the upper part of the egg, and passing down 
the sides, gradually fade and disappear before reaching the 
lower pole. The food-yolk even at this period preponderates 
in the lower part of the egg, and thus prevents the yolk- 
division taking place so rapidly as it does above. These 
furrows may be compared to two meridians on a globe; the 
next one (judging from the analogy of the Frog) represents 
the equatorial furrow in Amphioxus, but, for the reason above 


38 W. B, SCOTT AND HENRY F. OSBORN. 


stated, it is much nearer to the upper pole than to the lower 
and this gives at once the distinction of larger and smaller 
blastomeres. The smaller blastomeres grow round the ovum 
over the larger, and bear the same relation to them as they 
dointhe Frog. The segmentation cavity appears early, and 
from the very first its roof is only one cell thick, just as in 
the case of the Lamprey. As we shall see later the epiblast 
is at first composed of one layer, and hence the roof of the 
cavity is covered by epiblast only. In the Elasmobranch 
Fishes the roof of the cavity is formed by lower layer cells 
also, and this Mr. Balfour explains by the increase in the 
quantity of food-yolk in the cells, compelling them to 
creep up the sides of the cavity. Although there is propor- 
tionately more food material in the Newt’s egg than in that 
of the Frog the increase is relatively-ssmall and does not 
affect the position of the cells. “The only difference between 
the two at this stage consists in the fact that the roof of the 
cavity in the Frog is two or more cells thick, and in the 
Newt only one. In short, the ovum of the latter resembles the 
morula of Amphioxus with a large amount of food material 
stored away in its lower part. Judging from the descrip- 
tions of Calberla, it is in no way different from the ovum of 
Petromyzon of corresponding age. The floor of the segmen- 
tation cavity, as in all ova which contain food-yolk, is formed 
by the upper layer of yolk-cells from which, eventually, 
the ventral epithelium of the alimentary canal is in part 
derived. 

The next step in development is, as in the Batrachians, a 
process of invagination, and, as in them, it is an unsym- 
metrical invagination. The disturbing cause is in both cases 
the presence of the food-yolk below. Owing to the fact that 
the food to be made available must be placed upon the ventral 
side of the body, the invagination must in this region take 
place very slowly or not at all. By this simple considera- 
tion Mr. Balfour explains the unsymmetrical gastrula of the 
higher Vertebrates. 

At the period when our study of the two lower layers 
proper begins, segmentation is complete; the lips of the 
blastopore are rapidly nearing each other ; the epiblast con- 
sists of a single layer of partly columnar, partly wedge- 
shaped, cells, and has already in great measure attained those 
characters which persist throughout several of the following 
stages. 

At the lip of the invagination (see Plate IV, fig. 2) there 
is a decided swelling produced, in part by a lengthening, in 
part by a reduplication of the cells, a histological change 


EARLY DEVELOPMENT OF THE COMMON NEWT, 39 


analogous to that which has been pointed out in the so-called 
embryonic rim in the Elasmobranchs.! The cells have 
a radiated arrangement, losing as they are reflected inwards 
their columnar character and becoming more spindle-shaped. 
As they approach the inner side of the lip they are quadrate, 
then oblong, then columnar, their outer ends abutting 
against the inner ends of the long epiblast cells. As the 
sections pass into the lateral region of the embryo, this rela- 
tion is lost, and confluent with the forming hypoblast cells 
are the parent mesoblast cells. The latter may fairly be 
considered to arise actually from the point of invagination 
and not as a secondary splitting off from the hypoblast on 
either side. 

Two longitudinal sections of an embryo at this period 
have been figured in Plate IV, figs. 2 and 3. Fig.2 represents 
a section passing through the median line, and those changes 
in the epiblast at the lip of the blastopore which have been 
just referred to, may be followed. The alimentary canal has 
not proceeded far forwards, but the cells of the upper yolk 
are plainly forming the future hypoblast cells. The segmen- 
tation cavity is being pressed downwards; the section is in 
the median line behind and out of the median line in front. 
The reverse is true of the succeeding section (fig. 3), which 
represents the growth of the mesoblast at the sides of the 
invagination and the actual forward progress of the alimen- 
tary canal in the middle line. It illustrates the position and 
advancing obliteration of the segmentation cavity. Compar- 
ing the two sections, a very fair idea can be formed of the 
advance of the embryo in the early part of the stage (a). 

The process at the sides of the median line in Triton is 
then homologous to that which Gotte® represents as occur- 
ring in the median line in Bombinator, a construction which 
aids him in carrying out his peculiar views of the development 
of the notochord from the mesoblast. 

Calberla,? on the contrary, describes as the immediate 
result of invagination, in Rana temporaria, the primary 
entoderm. This does not split in the median line, while at 
the sides it splits soon after formation, to give rise to the 
lateral plates of mesoderm. A fuller notice of his views 


will be given later. 


1 Vide Balfour, ‘ Elasmobranch Fishes,’ chap. ii, p. 43. 
2 Vide Alexander Goette, ‘‘ Entwickelungsgeschichte der Unke,” ‘ Atlas,’ 


Tafel. ii. 
3 KE. Calberla, “ Zur Entwickelung des Medullarrohres und der Chorda- 


dorsalis der Teliostier und Petromyzonteu,” p. 261, ‘ Morphologischen Jahr- 
buch,’ 3, 1877. 


4.0 W. B. SCOTT AND HENRY F, OSBORN. 


Our sections do not wholly accord with the observations 
of either of the above, for if it is clear that the invagination 
gives rise in the median line toa single layer of cells, it is 
equally clear that at the sides it gives rise to a double layer, 
namely, of mesoblast as well as hypoblast. 

The process in Triton agrees then more closely with that 
occurring in the Elasmobranch Fishes,! where the lower Jayer 
cells, confluent with the reflected epiblast on either side of 
the axial line, form a layer of spherical cells above and co- 
lumnar cells below, and the former is ultimately separated off 
as the mesoblast proper, while in the axial line the lower 
layer cells give rise simply to a columnar layer. 

Now, turning to the transverse section of a Triton embryo 
Stage a (see Plate IV, fig. 4) we find that it adds still 
further probability to this view, for the relations of the 
layers fully accord with the above interpretation of the 
invagination. 

Now, as concerns the further growth of the mesoblast, it 
results from the foregoing conclusions concerning the hypo- 
blast that the mesoblast is never present across the axial 
line in the early stages. In transverse sections of Stage a 
it appears as two lateral plates extending on either side to 
a point just above the side limits of the alimentary canal. 
The layer where it is nearest the alimentary canal consists of 
small round cells, one or two deep, which can be readily dis- 
tinguished from the adjacent hypoblast. These are the 
cells which we have just referred to as having resulted from 
invagination, and we shall speak of them hereafter as the 
primary mesoblast cells. 

In conclusion, all the observations we have made favour 
the above interpretation, while none in any way disprove it. 

Thus, at once three important distinctions are established 
between the development of the layers at the point of invagina- 
tion in Triton and Bombinator, if we accept in full Dr. Gotte’s 
investigations of the latter. First: in Triton there is a 
decided thickening of the single layered epiblast as it 
approaches the point of invagination. In Bombinator there 
is none. Second: the resulting hypoblast in the axial line 
is in direct contact with the epiblast. There is no inter- 
vening mesoblast as in Bombinator. Third: the mesoblast 
is found in Triton as two lateral plates, and is not con- 
tinuous across the middle. 

These observations, coupled with those of Calberla, we think 
leave little doubt that Gotte has mistaken the upper hypo- 
blast cells for mesoblast, and thus at the start fallen into an 

1 Vide ¥. M. Balfour, ‘ Elasmobranch Fishes,’ p. 49. 


EARLY DEVELOPMENT OF THE COMMON NEWT. 4j 


error which involves some of his subsequent conclusions in 
donbt. 

Having thus briefly considered the origin of the two inner 
layers, as related to the phenomena of invagination, we shall 
return to the history of the epiblast from the beginning, and 
resume our discussion of the mesoblast and hypoblast in the 
subsequent pages. 


General Features of the Epiblast. 


When the epiblast can first properly be said to be formed, 
it consists of a single layer of very large quadrate cells, with 
large clear nuclei. Inthe next stage, when the invagination 
first commences, the cells have somewhat lengthened out, 
but are still very broad (Plate IV, fig. 1). When the in- 
vagination has progressed considerably, and the segmenta- 
tion cavity has been much narrowed, we find that the cells 
have assumed the condition which they retain for some time 
after this. They are long, narrow, and columnar; most of 
them can be traced through the layer from one surface to 
the other without any change of size, although here and 
there several may be seen which have a wedge-shape, and 
alternate arrangement with their neighbours. The nuclei, 
however, are arranged in two rows, like those of the Elasmo- 
branch epiblast. In general appearance, up to this time, 
the epiblast is more like that of Petromyzon than of any 
embryo which we have seen,’ but the arrangement of the 
cells is somewhat more regular. For a short time, indeed, 
the appearance of the two is almost identical, but in the 
Newt the cells speedily become narrower, and more columnar 
in character, and the nuclei assume the alternate arrange- 
ment which is only faintly indicated in the Lamprey. 
During Stage a, when the medullary groove has begun to 
make its appearance, the middle line of the dorsal epiblast, 
exhibits a decided thinning to form the groove (Plate IV, 
fig. 4). But this grove is not at this period, nor do we find 
it afterwards, nearly so deep or so wide as it is in the Elas- 
mobranchs.’ 

The next change of importance takes place during Stage 
B (Plate IV, fig. 5), when the medullary folds are well 
formed. These folds are caused by the multiplication of 
cells of the epiblast, which here becomes much thickened, 
Although the folds are several cells thick they show no indica- 

* tion of being separated into different layers. With the excep- 

1 See a paper by Calberla, ‘ Morph. Jahrbuch,’ 1877, 3, taf. xii, fig. 7. 

* Balfour, loc. cit., plate iv, fig. 8 a. 


42 w. B. SCOTT AND HENRY F. OSBORN. 


tion of the medullary plate the remainder of the epiblast 
shows no especial change from the condition seen in the 
preceding stage. In the medullary plate, on each side of the 
middle line, is a low rounded ridge (Plate IV, fig. 5), which 
is formed by the increase in length of the epiblast cells, and 
perhaps partly also by the wedging in of the mesoblast along 
these two lines. 

The condition of the spinal cord at this period recalls the 
the condition of the same organ in the Batrachia of this 
age. For in the latter the nervous and epidermic layers 
fuse together into one indiscriminate mass, and do not 
separate again till much later. This separation takes place 
for the first time in Triton, not far from the age im which 
it reappears in the Batrachia. During Stage c sudden and 
rapid changes make their appearance. The medullary folds 
are now very prominent, and are composed of numerous 
elongated spindle- aud wedge-shaped cells, while in many 
places the medullary plate shows a commencement of 
the same process (Plate IV, fig. 6). But as yet in 
neither of these regions are any distinct layers to be seen. 
The lateral epiblast is just beginning to separate into two 
layers ; the process commences immediately outside of the 
medullary folds, and spreads down the sides of the embryo, 
until it has been completed all around (fig. 6). Plate V, 
fig. 9, shows a drawing on a larger scale of the point where 
such changes are going on most actively. Even with the 
aid of this we have not thoroughly satisfied ourselves as 
to the exact manner in which these changes are accomplished. 
Three suppositions may be made with regard to it—(1) that 
the upper layer splits off from the lower by a process of 
cell division ; (2) that the wedge-shaped cells draw in their 
edges, and lying in alternate arrangement come to make two 
rows, one above the other; (3) that both of these have their 
share in the process. On the whole we rather incline to the 
latter opinion. In favour of the alternate decrement of 
length is the fact that for some time preceding the separa- 
tion the nuclei of the cells are arranged in two alternate 
rows, very much as in the Elasmobranchs, while such an 
appearance as shown at the point a, fig. 9, looks as if it 
could only be cell division. 

Turning to Stage p (Plate IV, fig. 7), we find that in 
the trunk region the medullary canal is completely closed, 
and the division of the epiblast carried entirely around the 
embryo, giving us two well-marked layers. These are com- 
posed of quadrate, somewhat flattened cells, of nearly equal 
size in both layers. The cells composing the spinal cord 


EARLY DEVELOPMENT OF THE COMMON NEWT, 43 


are numerous, elongated, wedge- or spindle-shaped; but 
even yet there is no indication of distinct layers. 

As in the Bird, the Mammal, and the Elasmobranch Fish, 
the epithelium lining the spinal canal does not become dif- 
ferentiated till a considerably later period. 

As a whole the spinal cord is now a hollow cylinder with 
very thick walls and a very small lumen. It presents a 
transversely oval section, and is somewhat indented on its 
lower surface by the pressure arising from the notochord. 
The epiblast has met and coalesced along the middle line 
above the canal, though a slight groove still shows the line 
of union. 

From this time forward the outer layer of the general 
epiblast becomes flatter and flatter, while the inner layer 
grows more columnar. But in those parts of the skin which 
cover the brain both layers are composed of very much 
flattened cells (Pl. V, fig. 13). The inner or mucous layer, 
when once formed, is the active layer, and from it alone such 
structures as the lens of the eye are derived. 

The primitive condition of the epiblast in Triton is an 
extremely interesting one, presenting in a somewhat un- 
expected manner great differences from that of the Frog. As 
is well known, in the latter animal the epiblast is double- 
layered from an extremely early period, the roof of the seg- 
mentation cavity being formed by two layers of cells, and by 
the time of invagination there is an outer stratum of a single 
row of flattened cells and an inner stratum of several rows of 
rounded cells, the epidermic aud nervous layers of Stricker. 
** Both strata have a share in forming the general epiblast, 
and though eventually they partially fuse together, there can 
be little doubt that the horny layer of the adult epiblast, 
when such can be distinguished, is derived from the epi- 
dermic layer of the embryo, and the mucous layer of the 
epiblast from the embryonic nervous layer. Both layers of 
the epiblast assist in the formation of the cerebro-spinal 
nervous system, and they also at first fuse together, though 
the epidermic layer probably separates itself again as the 
central epithelium of the spinal canal.” (Balfour, loc. 
cit., p. 99.) 

All this is very different from what we seein Triton. At 
first the epiblast is of one layer, and so remains for a con- 
siderable time ; the mucous layer, when formed, consists of a 
single stratum of more or less columnar cells, and the epi- 
thelium of the spinal cord appears for the first time at a much 
later period. In short, the condition of the epiblast, except 
in the last respect, is more like that of Pelromyzon than that 


44 W. B. SCOTT AND HENRY F. OSBORN. 


of the Batrachia. It is, as might be expected, intermediate 
between the two types in many ways. In the Lamprey the 
division into two layers does not occur until a comparatively 
late period, some time after the larva has been hatched, while 
in the Newt it occurs as early as Stage c. In the Frog it 
is found from the first. Another respect in which the Newt 
is intermediate. is the histological character of the layers. 
The Elasmobranch Fishes in this respect present an inter- 
mediate condition between the Lamprey and the Newt. In 
them also the epiblast is primarily single; the first change 
consists in the part which will give rise to the central 
nervous system, becoming several cells thick, but presenting 
no distinction into two layers. Eventually, later than in the 
Newt, earlier than in the Lamprey, the epiblast divides into 
mucous and epidermic layers. Both layers seem to enter into 
the formation of the organs of sense, while in the Amphibians 
the sense organs are formed exclusively, or almost so from the 
mucous layer, and in the Lamprey they are derived from the 
epiblast before its division into the layers. 

These facts put us in a somewhat favorable position for 
the solution of the question as to whether the single- or 
double-layered epiblast is the primitive condition. We are 
decidedly of the opinion that the conclusion drawn by Mr. Bal- 
four on p. 100 of his book on the Elasmobranchs is the correct 
one, viz. that the single-layered epiblast is the more primitive 
condition. He was not aware at that time of the difference 
existing between the Frog and the Newt in this regard, and 
so attributed the double layer to the Amphibians generally. 
But, as we have seen, it is confined to the Batrachians, a 
much more restricted group, and is, perhaps, also found in 
Osseous Fishes. Besides these it is found in no other groups 
of the animal kingdom, and, as Mr. Balfour points out, it is 
more probable that a particular feature of development should 
be thrown back to an earlier period than for the distinction 
between the two layers to be absolutely lost, and then to 
reappear ata later stage. This d prior? consideration receives 
a great deal of support from the facts of the development of 
the Newt. By its aid we are enabled to arrange a series of 
steps of advancing differentiation of the epiblast from Amphi- 
oxus through the Marsipobranchs, the Elasmobranchs, and 
the Newt, to the Batrachians. 

The steps of this progression have been already stated, but 
it may be well to summarise them. (1.) Amphioxus has an 
epiblast consisting at first of short columnar cells in a single 
row. These afterwards begin to flatten out, and in the adult 
are very much flattened, but never constitute more than a 


EARLY DEVELOPMENT OF THE COMMON NEWYD. 45 


single row. The medullary plate is the only epiblastic 
development which consists of more than one row of cells. 
This fact alone is of considerable weight in the question we 
are considering ; and it should be borne in mind throughout 
the discussion that, in the most primitive vertebrate known, 
the epiblast is permanently single-layered. Into the peculiar 
method of the formation of the cerebro-spinal axis we need 
not enter. 

(2.) Inthe Lamprey the epiblast does not divide until very 
late; in fact, not before the embryo has for some time been 
hatched (see Calberla, loc. cit., p. 264). This change 
takes place, however, in the region of the spinal cord before 
that organ has been formed, just as is the case in Amphioxus. 
The development of the nervous axis presents some pecu- 
liarities of a secondary nature. The sense organs are formed 
from the undivided epiblast. 

(3.) The epiblast in the Elasmobranch Fishes separates 
into two layers much earlier than it does in the Lamprey, but 
still comparatively late in embryonic life, some time after the 
medullary canal has been completely closed, and several of 
the visceral clefts have appeared. According to Mr. Balfour 
it takes place at a stage slightly younger than K. The two 
layers are at first composed of flattened cells, but those of 
the inner stratum soon become columnar. ‘‘ Both layers 
apparently enter into the formation of the organs of sense.” 

(4.) In Triton the epiblast, though at first single, divides 
into its two parts at a very early stage, some time before 
the closing of the medullary canal (Stage c). When once 
formed the mucous layer becomes the active one and enters 
almost exclusively into the formation of the sense organs. 
So far as we are aware this is the only case as yet known in 
which there is a primitively single epiblast dividing early 
and delegating all its activity to one layer. It shows an 
approximation to the state of things found in the Frog. 

(5.) In the Batrachia this is carried one step further and 
the two layers are distinguishable from the very first, even 
the roof of the segmentation cavity being double. The 
niucous or nervous layer, as in the Newt, enters alone into 
the formation of the organs of sense, &c. In short, almost 
the only difference in the matter of epiblast between the two 
classes of Amphibia lies in the é¢me of its division. 

Now, we are very far from asserting that these forms 
we have been considering represent the line of descent of the 
Batrachia; but we are decidedly of the opinion that they 
exhibit the steps of the process by which the epiblast of 
that group has reached its present complication. For 


46 W. B. SCOTT AND HENRY F. OSBORN. 


this reason we are forced to the conclusion that even the 
early condition of the epiblast in the Batrachia is a secondary 
modification, and that the primitive condition of the layer ts 
single. 

As opposed to this conclusion may be adduced the fact that 
in the spinal cord of the Batrachia the two layers at first 
fuse together and at a later time reappear, as if the double- 
layered condition were a primary, the single-layered a 
secondary, and the reappearing double layer a tertiary stage 
in development. And further, that the first stage has been 
retained only in the Batrachia and (?) Osseous Fishes, and lost 
in other known vertebrates. But this appears unlikely, and 
standing entirely by itself, the above-mentioned fact cannot 
be considered to have any great value. 


The Hypoblast. 


We shall now continue the history of the hypoblast from 
Stage A onwards, until the development of the notochord. 
The embryo at this stage (see Pl. IV, fig. 4) is still spherical. 
In the section figured, which is in the anterior region of the 
embyro, the alimentary canal is broad and low, lined above 
by a deep single layer of columnar hypoblast cells. These 
are broader and longer than the epiblast cells above them, 
with nuclei of a spherical rather than oval shape. They are 
in contact with the epiblast broadly across the middle line, but 
at the sides, just below the two slight folds on either side of 
the medullary groove, the mesoblast begins to intervene as a 
single layer of small cells. Beneath these the hypoblast 
cells lose their columnar shape, and becoming more quadrate 
are gradually reflected around the sides of the alimentary 
canal, becoming continuous on the one hand with the quad- 
rate yolk cells lining the alimentary canal below, on the 
other with the cells bounding the great mass of yolk. This 
continuity has been carefully represented in Pl. IV, fig. 4. 
Where the invagination cells cease would be difficult to state, 
owing to the fact that the bending down at the sides is a 
gradual process partly dependent upon the growth of the 
mesoblast. 

The hypoblast can now be classed according to its develop- 
ment under two heads. (a.) The cells above the alimentary 
canal, which have arisen from invagination and are con- 
tinuous with the reflected epiblast at the blastopore. This 
we shall call the invagination hypoblast. (6.) Those cells 
lining the alimentary canal below and those immediately 
bounding the yolk elsewhere, which arise by histological 


EARLY DEVELOPMENT OF THE COMMON NEWT. 4.7 


changes in the yolk cells proper. We shall refer to this as 
the yolk hypoblast. 

The growth of the former class has been already con- 
sidered in full. The latter arises by a slow process of meta- 
morphosis in the peripheral yolk cells. The changes are 
not difficult to follow. The square yolk cells split as they 
approach the surface into long columnar or oblong cells, and 
at the same time a change takes place in the yolk spherules 
with which they are loaded, so that they show a greater 
avidity for the staining fluid. ‘The large spherical nuclei of 
the yolk cells give place to the characteristic oval nuclei of 
the hypoblast. These primitive hypoblast cells assume more 
regular proportions as development proceeds. In the split- 
ting off of the mesoblast which soon follows, fresh cells are 
constantly supplied from the yolk. 

A further notice of Calberla’s! views upon these points will 
perhaps not be out of place here. He considered the Lam- 
prey embryo immediately after invagination to consist of two 
layers, the primary entoderm and the ectoderm. The former 
divides everywhere, except across the axial line, into the 
secondary entoderm and the mesoderm. Across the axial line 
the primary entoderm remains intact. He does not admit that 
the mesoderm arises even in part by invagination; but, still 
more important as it bears on the question under discussion, 
he does not include the outer yolk cells as part of the pri- 
mary entoderm. So what we shall consider hereafter as the 
lateral mesoblast, he concluded, was joint mesoblast and 
hypoblast, not allowing that the outer yolk cells formed a 
distinct layer. The comparison has been inserted because at 
this period of its history the Lamprey presents many points 
in common with the Newt. 

To resume the study of the hypoblast in Triton, it may 
be considered in the latter part of Stage c as forming a con- 
tinuous layer around the yolk and completely enclosing the 
alimentary canal. By Stage Ba very decided change has 
taken place (see Pl. IV, fig. 5). The section is in the head 
region where the alimentary tract has now reached a con- 
siderable size. ‘The hypoblast is now only in contact with 
the epiblast in the median line, although the connection is 
such a close one that the three or four cells, still adhering, im- 
pinge so closely as te form a decided indentation in the 
epiblast—a feature which has been previously noticed in the 
Elasmobranch Fishes. The middle cells have also elongated 
and narrowed considerably, while those at the sides remain 
shorter ; this results in a rounded upper outline. Laterally, 

Vide Ki. Calberla, loc. cit., on ‘ Petromyzon planeri, 


48 W. B. SCOTT AND HENRY F. OSBORN. 


they are still markedly continuous with the yolk hypoblast 
cells lining the alimentary canal and their lower margin 
arches upwards so as to form part of the lumen of the 
canal. This bending around of the hypoblast, which in 
Stage a -was almost a straight line, into an arch of cells, 
must be partly attributed to a mechanical cause, viz. the 
rapid ingrowth of the mesoblast plates. Whatever the exact 
cause of this change it is well to note that no vital altera- 
tion has as yet taken place—the change is one merely of 
position. Elsewhere the hypoblast shows no new features. 

Inasmuch as the interest in the hypoblast chiefly centres 
around the development of the notochord we shall con- 
sider the history of that organ by itself and complete the 
hypoblast later. 


The . Mesoblast. 


It is evident from transverse sections in the latter part of 
Stage a (see Pl. IV, fig. 4) that the lateral plates of meso- 
blast have already attained a considerable thickness. At the 
junction of the invagination with the yolk hypoblast they 
are three or four cells deep, thinning out rapidly at the sides. 
In the anterior sections they barely extend below the middle, 
while behind they meet as a single layer of cells at the 
bottom, thus encircling the hypoblast except in the axial 
line above. 

The lateral downward growth of the mesoblast in Triton is 
plainly not from the epiblast, for the epiblast has by this time 
formed a distinctly bounded single layer. There remain two 
modes in which it may % great part arise, (a) from the 
hypoblast ; (6) independently of the hypoblast, from the yolk. 
This is of course excluding the mesoblast in the region of the 
alimentary canal which accompanies the process of invagina- 
tion. If we consider, as we have reason to do from the analogy 
of the Frog, that the cells bounding the yolk form the primi- 
tive yolk hypoblast layer, we can only accept the former hypo- 
thesis. In the anterior section of Stage a the cells bound- 
ing the yolk below are as unquestionably hypoblastic as those 
bounding it above and at the sides. In other words, the 
hypoblast has formed as a distinct layer in contact with the 
epiblast below, before the mesoblast has appeared below at 
all. Moreover, at the sides, the down growth of the meso- 
blast is preceded plainly by a splitting off of the outer portion 
of the yolk hypoblast into large quadrate cells, and these in 
turn are seen in the process of subdivision. Although this 
growth seems to be at the expense of the hypoblast, it cannot 
be considered to arise altogether independently of the down- 


EARLY DEVELOPMENT OF THE COMMON NEWT. 49 


growth of the invagination plates by a process of cell division, 
for the mesoblast does not arise at separate points, capping 
the hypoblast, but in direct continuity with the invagination 
mesoblast. | 

In the Elasmobranch Fishes, in which the origin of the 
mesoblast has been carefully observed, there is no doubt 
that this layer arises as two lateral masses, splitting off 
from the hypoblast at the same time that the latter arises as 
a distinct stratum from the lower layer cells. Here, however, 
the lateral plates do not form a continuous layer with the 
mesoblast which occasionally arises at the reflection of the 
epiblast at the sides, but are distinct from it. 

Calberla,! as previously stated, explains the growth of the 
mesoderm (mesoblast) in the Lamprey, as an early splitting 
of the outer portion of the primary endoterm. This view 
fully confirms our interpretation of the lateral growth in 
Triton. 

In Kowalevsky’s earlier researches upon Amphioxus he 
fell into the error of supposing the mesoblast of double 
origin, hypoblastic and epiblastic, an error which he cor- 
rected later? by attributing this layer to a constriction off 
from the hypoblast, which occurs subsequent to the forma- 
tion of the notochord. The simple invagination does not 
give rise to any but the two primitive layers. There can 
now be no doubt that the formation of the mesoblast is in 
all types a secondary phenomenon which is retarded in the 
simpler forms, and hastened in the more complex into an 
earlier period of development. 

To review the features noticed in Stage A. The meso- 
blast arises by invagination as two lateral plates, and is 
never found across the median line. Subsequent growth is 
partly by cell division of these plates; mostly, however, at 
the expense of the hypoblast. The most rapid development 
is posteriorly, both in respect to thickness and downward 
growth. There is no tendency to split into somatic and 
splanchnic layers. By Stage B the mesoblast shows a very 
marked progress. It is now thickest immediately below the 
medullary plates, and causes that upward curve in the out- 
line of the epiblast previously mentioned (Plate IV, fig. 5). 
At the same time the lateral plates have approached each 
other, bending the hypoblast downwards, so that now it is 
continuous with the epiblast only in the median line. ‘The 
section figured is in the anterior part of the embryo near the 
head region. ‘The cells appear larger than in the last stage, 

1K. Calberla, loc. cit. 

? Vide A. Kowalevsky, ‘Archiv. fur Micros. Anatomie.’ Band 13, p. 191. 

4: 


50 W. B. SCOTT AND HENRY F. OSBORN. 


near the axial line they are crowded together irregularly, 
but at either side the splitting into two single-celled layers 
begins to be evident. ‘This splitting begins anteriorly and 
proceeds slowly backwards. In the posterior sections of the 
same embryo it is barely evident, although the cells show a 
tendency to arrange theinselves in tworows. Plate IV, fig. 6, 
represents a section from the trunk region during Stage c, 
and shows that the splitting of the mesoblast extends slowly 
backwards. In this region the layer is now thinner than it 
is forwards, although the reverse of this is true of Stage a, 
where the mesoblast is thickest posteriorly. The proximal 
cells now begin to arrange themselves radially around the 
vertebral portion of the future body cavity, closely im- 
pinging against the epiblast, and tending to grow in above 
the primitive notochord. The body cavity does not extend 
beyond the medullary folds in this embryo, for here the two 
rows of cells suddenly terminate in a single row bending 
around the sides. In other respects the mesoblast shows no 
new features until Stage D. Sections of an embryo, during 
the latter part of Stage p, show that the neural canal has 
completely closed. The section figured in Plate IV, fig. 7, 
is in the anterior trunk region, here the mesoblast appears 
as two great triangular muscle plates, expanding above so 
as to fill the space formed by the fusion of the medullary 
canal, and enclosing the large body cavity. The two layers 
now extend completely around the embryo, but have not 
separated except in the upper region. In Stage F the divi- 
sion into somites has begun. 

To conclude, there is one feature in the development of 
the mesoblast, which argues strongly for the fact that, meso- 
blastic invagination being begun, lateral growth sets in at 
once; that is, the cells formed by invagination are immedi- 
ately supplemented by those growing down at the sides, of 
hypoblastic (yolk cell) origin. As evidence of this we find 
the mesoblast of the posterior sections meeting in the median 
line below, before it even reaches the ventral region anteriorly. 
In this single respect, the mesoblast develops more rapidly 
behind than in front. Subsequent to the formation of the 
alimentary canal, the greater energy of the embryo is 
directed to the head region, and all following growth is 
from before backwards. This is true of the thickening 
of the lateral plates, of the splitting into two layers, of the 
formation of the body cavity, and of the subsequent division 
into somites. 


EARLY DEVELOPMENT OF THE COMMON NEWT. 5) 


The Notochord. 


In our description of the hypoblast, we considered the 
layer as classed under two heads, the invagination hypoblast, 
and the yolk hypoblast; it is with the former that the 
development cf the notochord is concerned. The cells lying 
during Stage B between the mesoblast plates may be con- 
sidered the primitive notochordal cells. 

The first indication of the growth of the notochord in 
Triton -(see Plate IV, fig. 5), is the tendency of the cells 
to take a radiated arrangement. We may now at the out- 
set, point out three prominent features. First, the hypo 
blast consists of a single layer of columnar cells running 
from the epiblast above to the alimentary canal below. 
Second, these cells may be identified with the broad band of 
invagination cells which in Stage a were all in contact with 
the epiblast. They have been bent down by the ingrowth 
of the mesoblast above. Third, these cells are directly con- 
tinuous at the sides with the yolk hypoblast. 

In the Lamprey,! Petromyzon planeri, the relations of 
the hypoblast at this point to the epiblast and mesoblast are 
practically the same. ‘There is the same close and broad 
contact with the epiblast, and the cells are of the same 
relative size. Here, as in Triton, the primary or invagina- 
tion cells are alone concerned in the origin of the notochord. 

In the Frog (Rana temporaria) the primitive condition of 
the notochord is a great cubical mass of small cells, con- 
fluent with the epiblast above, and with the mesoblast at 
the sides. These do not all enter into the formation of the 
notochord, however, for at the time this organ begins to be 
coustricted off, the lower cells form a hypoblastic lining to 
the alimentary canal. Gdtte’s account of the first appear- 
ance of the notochord in the Frog (Bombinator igneus) 
differs widely, owing to the fact that he has mistaken the 
upper hypoblast cells for the mesoblast. 

In the Elasmobranch Fishes® the arrangement is analo- 
gous, for the whole layer with the exception of a thin line of 
cells over the alimentary canal, enters into the notochord. 
The cells are at no time so widely in contact with the 
epiblast as in Triton; so the change preceding the formation 
of the notochord consists, first, in the lengthening, and then 
splitting of the cells into two lines placed end to end. The 
lower line thus formed is, however, mostly absorbed in the 

' Vide K. Calberla, loc. cit. 


2 Vide ¥. Calberla, loc. cit., p. 260. 
3 Vide Balfour, loc. cit., p. 93. 


52 WwW, B. SCOTT AND HENRY F. OSBORN. 


formation of the organ, and is not, as in Rana temporaria, 
wholly expended in forming the upper layer of the alimen- 
tary canal. To return to Triton, it is well to notice here 
that the upper boundary of the alimentary canal is formed 
by the cells which will give rise to the notochord, and that 
the latter at this period actually contains part of the lumen 
of the canal. 

Following the notochord into the succeeding stage, we 
find no marked changes (Pl. IV, fig. 6). The section taken 
from the middle region of the embryo presents much the 
same appearance. From this we infer that in common with 
the other organs, the notochord develops more rapidly for- 
wards, and that the backward development is a slow one, 
for in Stage c the notochord is but little more advanced in 
the middle region of the embryo than it is in the anterior 
region in the preceding stage. The primitive features 
pointed out above remain constant. 

Unfortunately there is a gap in our sections here, at least 
we have none by which we can trace the histological changes 
from the simple fold of hypoblast cells in Stage c, to the 
firm rod of radiating cells in the latter part of Stage D. 
There is no evidence of their splitting into two cells deep 
previous to this result as in the Lamprey and the Elasmo- 
branchs. The exact process beyond the ascertaining of this 
point is of little real importance. 

In Stage p (PI. IV, fig. 7) the relations of this organ are 
not much altered, it still impinges against the epiblast 
above, and partly bounds the alimentary canal below, but 
the continuity with the hypoblast has been broken off, and 
the line of demarcation is plainly marked by the different 
character of the cells. The notochordal cells are subquadrate 
in shape, about twelve in number in a transverse section, 
and are arranged around a centre of their own. In other 
words, the notochord is now an independent body; at its 
sides below are the long narrow hypoblast cells which par- 
tially enclose it, and above are the mesoblast plates fully 
formed, which, however, show no tendency to sur- 
round it. 

The notochord is now larger than at any subsequent stage. 
In its formed or permanent condition, it persists as a close 
granular mass in which we can sometimes detect cell 
division, sometimes not. (See Pl. V, fig. 8; figs. 12 
and 13.) In Stage = an ingrowth of hypoblast below, cuts 
off its connection with the alimentary canal. In a much 
later period, Stage mM, it has a vacuolated appearance; a 
branching network of connective tissue supporting promi- 


EARLY DEVELOPMENT OF THE COMMON NEWT. 53 


nent nuclei, an appearance which has been noticed in 
many other forms (Pl. IV, fig. 15). 

This completes the interesting history of the development 
of the notochord. To summarise: The invagination hypo- 
blast cells are first continuous as a single layer, wholly 
across the median line; those farthest from the three central 
cells are gradually pushed down by the ingrowth of the 
mesoblast. There is no tendency to split below. They are 
further reflected around until the lateral cells meet, and the 
continuity with the hypoblast is broken. It still impinges 
against the epiblast above, and forms the upper boundary of 
the alimentary canal below. 

A comparison has already been instituted between the 
development of the notochord in Triton and its development 
in the Frog, the Lamprey, and the Elasmobranch Fishes. 
In important details the processes are very similar. ‘To 
carry the comparison a step further, in Amphioxus the noto- 
chord is differentiated from the hypoblast before the meso- 
blast has become constricted off, and at the time that the 
medullary plate is closing in above. 

Hensen has demonstrated, beyond doubt, that the noto- 
chord is of hypoblastic origin in the Guinea-pig; and that it 
probably arises in the same way in the Rabbit. Quite 
recently,” Mr. Balfour has shown that it has a similar deriva- 
tion in the Lizard, Lacerta muralis. 

In several respects the notochord arises in a simpler 
manner in Triton than in any of those forms in which the 
process has been clearly followed out. In that: first, the 
-cells do not reduplicate vertically, as in the Elasmobranchs 
and the Lamprey, previous to the formation of the organ; 
second, when the organ is completely formed, it still bounds 
the alimentary canal below, as in neither of the other forms 
nor in the Frog; third, no portion splits off subsequently to 
form the hypoblast layer bounding the canal above, this 
layer appears from the sides. 

It is difficult to judge from Kowalevsky’s description, 
whether the whole depth of the layer bounding the canal 
above is absorbed by the notochord, or whether the lower por- 
tion remains as an upper lining of the canal, and the upper 
portion alone enters into the notochord. If the latter is the 
case, the Newt presents the simplest notochordal develop- 
ment known. 

The evidence from all these forms points so strongly in 
one direction, as to amount almost to proof, that the study 


' Vide Kowalevsky, loc. cit. 
2 Vide ¥. M. Balfour, this Journal, Vol. XIX, p. 3, New Series. 


5A. W. B. SCOTT AND HENRY F, OSBORN, ~ 


of the more important types which have not as yet been 
observed, and the clearing up of the doubts which still 
envelop other types, will fix the derivation of the notochord 
from the hypoblast as a law, rather than as a feature posi- 
tive in some cases, and with an exceptional origin from the 
mesoblast in others. 


The Hypoblast. 


In Stage c the notochordal cells are continuous at the 
sides, with the layer of hypoblast lining the yolk (see PI. 
IV, fig. 6). In Stage p this continuity is completely 
broken, the layer appears as a long narrow row of cells, 
flattened against the sides of the notochord, but not enclos- 
ing it below. Elsewhere this layer shows no new features. 
In Stage E, however (see Pl. V, fig. 8), the cells have 
grown down and meet below, completely surrounding the 
alimentary canal and shutting it off from the notochord. This 
process is interesting, as it shows that, while the original 
upper lining is mainly absorbed by the notochord, the per- 
manent upper lining is formed from the yolk hypoblast cells, 
and that now almost the entire layer is formed of this 
secondary hypoblast, the bulk of the primary or invagina- 
tion hypoblast having gone to the notochord. The hypo- 
blast grows under the notochord, in much the same way in 
the Lamprey, but at a somewhat earlier stage. In most of 
the other forms there remains throughout, a thin layer of 
cells intervening between the notochord and the yolk. 


Body Cavity and Somites of the Head. 


As already mentioned, the growth of the mesoblast is 
from behind forward, and in Stage a (PI. IV, fig. 4) we see 
that in the head region the mesoblastic plates do not meet 
ventrally. ‘They gradually thin out forwards and end near 
the blind termination of the alimentary canal. At this 
period the mesoblast is quite thick, and is composed of nu- 
merous cells of spherical shape, but exhibits no tendency to 
become divided into somatic and splanchnic layers. In 
Stage B, however, the cells have arranged themselves into 
two layers, and quite a cavity has appeared between them 
(Pl. IV, fig. 5). As yet this change is confined to the head, 
and so there is a cavity in the head on each side of the mid- 
dle line, contained between the somatic and splanchnic 
layers of the mesoblast. ‘These cavities, therefore, are parts 
of the pleuro-peritoneal cavity, and when that is formed in 
the body, will be directly continuous with them. As in the 


EARLY DEVELOPMENT OF THE COMMON NEWT, 55 


Elasmobranch Fishes,' the cavity in the head is formed at 
a period considerably before that at which it appears in the 
body. These two head cavities have no communication 
with each other, as the mesoblast in the head is in two 
separate masses. A longitudinal horizontal section (Pl. V, 
fig. 10) through an embryo slightly older than F shows this 
cavity (pp.) as an undivided slit bounded by columnar 
mesoblast cells. But when the first visceral cleft appears as 
an outgrowth from the hypoblast of the throat to join the 
external skin, this cavity is necessarily separated into two 
portions, one behind and one in front of the cleft. This 
cleft in the latest stages we have been able to observe never 
pierces the skin, but it lies close to it and so divides the me- 
soblast. The second cleft divides the cavity into three sec- 
tions, and each succeeding one adds a fresh segment to the 
number. Of course this number is not so great as it is in 
the Elasmobranch Fishes. 

The section in front of the first cleft presents some features 
which demand attention. It grows forward and becomes 
_ divided spontaneously into two portions, one of which lies 
close to the optic vesicle (Pl. V, fig. 11), and entirely in 
front of the mouth, while the second (2 pp.) is enclosed alto- 
gether in the mandibular arch. The first aortic arch (laa) 
runs between these two sections and somewhat dorsal to 
them. We have not been able to make any satisfactory 
observations upon their relation to the branches of the 
fifth nerve, but from their position it seems in every way 
probable that they have much the same relations as those de- 
scribed by Mr. Balfour in the Elasmobranch Fishes. The 
first division shows a small lumen surrounded by a layer of 
short columnar cells; in longitudinal vertical sections (PI. 
V, fig. 11, 1 pp.), it has a somewhat oval shape; in trans- 
verse sections (fig. 13, pp.) it has a transversally elongated 
shape, and the cavity in these sections is seen to be largest 
toward the middle line. During no period as late as Stage L 
could we find any trace of a ventral union between the ante- 
rior segments of each side, such as occurs in the Elasmo- 
branchs. It may, however, occur later, as during Stage L 
they approach very closely, The second segment (Pl. X XI, 
fig. 11, 2 pp.) is considerably smaller than the first, and has 
a very small lumen. Its cavity also is lined with columnar 
cells, and forms a narrow slit running parallel to the first 
visceral cleft. ‘The mandibular aortic arch lies just anterior 
to it instead of close to its inner side as in the Elasmobranchs. 

The other segments of the head cavity lie in the visceral 

1 Balfour, loc. cit., p. 86. 


56 Ww. B. SCOTT AND HENRY F. OSBORN. 


arches, and show narrow cavities lined by columnar epithe- 
hum (Pl. XXI, fig. 12, 3 pp). They present no features of 
especial importance. We have not followed out the subse- 
quent development of these segments, but in all probability 
their cells become transformed into muscle cells. 

In the foregoing description there will be observed a very 
close similarity to what has been described for the Elasmo- 
branchs ; in fact, with some minor exceptions, and the one 
important one of the non-communication of the first pair of 
segments, Mr. Balfour’s descriptions will apply equally well 
to our specimens. This is of the more interest, for 
Triton in this respect is very much more like the Elasmo- 
branchs than it is like the Batrachians; a fact which is 
somewhat remarkable. In the Batrachians so carefully in- 
vestigated by Dr. Gotte,’ there appears to be no head cavity 
formed at any period. On the other hand, two series of 
segments, an inner and an outer series, become formed, and 
are believed by Dr. Gotte to correspond respectively to the 
vertebral and lateral plates of mesoblast which are developed 
in the trunk. ‘The internal segments resemble the proto- 
vertebre in shape, but are smaller; their walls develop into 
muscular fibres and represent the anterior continuation of 
the dorsal muscles. The external segments are contained in 
the visceral arches. In the anterior division of the head 
(Gotte’s Vorderkopf) there is only one pair of segments, as 
the division of the segment in front of the first visceral cleft 
does not seem to occur ; the part contained in the mandibular 
arch is derived from the growth of the postero-lateral seg- 
ments. The most anterior segment of all gives rise, as in 
the Elasmobranchs, to the muscles of the eye. 

It is remarkable how very different all this is from the 
process observable in Triton. There are found in the pos- 
terior part of the head four segments which give rise to 
muscular fibres, as in Bombinator, and continue the dorsal 
muscle forwards. These may be equivalent to the four in- 
ternal segments of the head of Bombinator, but they have no 
ventral continuations. They are more to be compared with 
segmentsin the posterior part of the head of the Elasmobranchs. 
With regard to the latter, Mr. Balfour says (p. 209), ‘* All 
my efforts have hitherto failed to demonstrate any segmen- 
tation in the mesoblast of the head other than that indi- 
cated by the sections of the body-cavity before mentioned, 
but since these must be regarded as equivalent to muscle 
plates any further segmentation of mesoblast could not be 
anticipated; to this statement the posterior part of the 

1 Unke, pp. 203-208, 216-229. 


EARLY DEVELOPMENT OF THE COMMON NEWT, 57 


head forms an apparent exception. Not far behind the 
auditory involution, there are visible at the end of Period K 
a few longitudinal muscles, forming about three or four mus- 
cle plates, the ventral part of which is wanting. I have not 
the means of deciding whether they properly belong to the 
head or may not be a part of the trunk system of muscles 
which has to a certain extent overlapped the back of the 
head, but am inclined to accept the latter view.” The 
appearances here described are very much like those to be 
seen in Triton, and we are not in a position to pronounce 
any more decided judgment upon them, than upon those of 
the Elasmobranchs ; but taking into consideration Gdtte’s 
figures we are rather inclined to consider them the axial 
segments of which the plate containing the head cavity is 
the lateral part. The chief differences between the two 
types of Amphibians lie in the cavities themselves, and 
the number of segments in the anterior part of the 
head. 

Our researches do not, we regret to say, throw much 
new light upon that difficult morphological problem, the 
segmentation of the head. It is interesting to find that 
as in the Elasmobranchs there is one pre-oral segment, as 
might be expected to be the case if the head cavities afford 
any trustworthy guide to the number of head segments. Of 
course the number of postoral cavities is less than in the 
Elasmobranchs owing to the fewer gill clefts, but this is a 
feature which does not affect the question at issue. 

Of whatever value these facts in the development of the 
Newt are considered, we think that they favour the views 
expressed by Mr. Balfour in p. 216 of his book. For these 
head cavities, if of morphological importance, might be 
anticipated to be fairly constant in character. 


The Thyroid Body. 


Pl. V, fig. 12, represents the earliest condition of the thy- 
roid body which has fallen under our observation. In it we 
see that in the region of the mandibular arch there is a 
solid outgrowth of cells from the ventral wall of the alimen- 
tary cavity which has reached the inner layer of the epiblast. 
The latter has at the point of contact risen up slightly from. 
the external layer leaving a small triangular space between 
them. In the next stage (fig. 13), the inner layer of epi- 
blast has coalesced with the hypoblastic outgrowth and is 
discontinuous across the middle line. It is now difficult to 
determine where one layer begins and the other ends, so 

5 


58 W. B. SCOTT AND HENRY F, OSBORN. 


complete is their fusion. The external layer is never inter- 
rupted. Fig. 14 presents a rounded thickening of the fused 
mass, which is the next step in development. 

The latest stage we have (somewhat later than m) shows 
the gland separated from the epiblast (Pl. V, fig. 15) which 
is now continuous across the middle line, but still connected 
with the ventral wall of the esophagus by a cord of cells. 
The thyroid is now a solid cylindrical rod of considerable 
length, ending posteriorly near the ventral aorta ; the section 
shows an aortic arch (1 aa) cut through longitudinally. The 
gland consists of an outer or cortical layer of columnar cells 
arranged radially, and an inner small kernel of rounded 
cells. As yet there is no trace of a lumen, or any division 
into lobules. Further than this we have. not been able to 
follow its development, but have. no reason to suppose that 
it presents any great peculiarities. 

On the whole the thyroid body-of the Newt corresponds 
quite closely in position and mode of development to the 
same body in the Elasmobranch Fishes ; but there are some 
points of difference to which we should like to call par- 
ticular attention. (1.) Inthe latter the diverticulum of the 
hypoblast is hollow in front and solid behind at first. and 
only subsequently becomes solid throughout, while in Triton 
we have not been able to discover any stage which shows a 
hollow outgrowth. The solidity, however, does not occur 
from any confused mass of cells, but from the fact that the 
two sides of the diverticulum are pressed closely together 
(Pl. V, figs. 12 and 13). .Of course it is very possible that 
we have missed a stage in which the outgrowth was hollow ; 
but if that is the case that condition must be a very tran- 
sitory one. The difference is only one of detail in any case. 
(2.) Of much more importance is the fact that in the Elas- 
mobranchs there is never found any indication of continuity 
between the hypoblast-and epiblast, which at this period is 
still single layered. But the diverticulum is pressed very 
closely against the epiblast, presenting just the appearance 
of the first visceral cleft which does not perforate the skin.! 
(We do not wish to intimate by this comparison an opinion 
that the thyroid is a modified visceral cleft, because all diver- 
ticula from the throat to the external skin must look more 
or less alike.) 

The account given by Dr. Gotte? of the development of 
the thyroid in Bombinator is still more like our account 
than is that given by Mr. Balfour of the Elasmobranchs. 


1 Balfour, loc. cit., Plate XIV, fig. 5a, p. 223-5. 
2 Loc. cit., p. 667. 


EARLY DEVELOPMENT OF THE COMMON NEWT. 59 


In Bombinator the thyroid “is formed from a pit of the 
hypoblast, which persists as the remains of an early depres- 
sion of the hypoblast behind the mandibular arch, produced 
by a fusion of the epiblast and hypoblast’’ (Taf. vii, fig. 127- 
130,and Taf. xiii, xv, xvi, figs. 292 and 293.) At first it is 
connected anteriorly with the median division line which 
bisects that arch; after the disappearance of this the rudi- 
ment of the thyroid appears as a funnel-shaped diverticulum 
of the hypoblast and is free below. The fusion between thetwo 
layers, which in Triton persists for a considerable period and 
is seen throughout the length of the gland, here is confined 
to the anterior end,-and remains only a short time. 

W. Miller, in his account of the development of the thy- 
roid body! in Rana temporaria, does not give any figures or 
descriptions leading us to suppose that he has observed this 
continuity of the layers. 

We must confess that we ourselves are very much puzzled 
by the fusion of the epiblast and hypoblast at this point, and 
are unable to give any morphological explanation of its 
meaning. Is it not just possible that it may represent some 
shifting in the position of the mouth ? but if so, we shall be 
obliged to abandon, for this form at least, the homology of 
the thyroid body with the endostyle of the Ascidians. We 
mention it with the hope of directing the attention of some 
morphologist, who will clear the matter up, to this curious 
and unexplained feature. 

It may be of use to give a brief summary of the points 
which we have endeavoured to establish in this paper, before 
passing on to consider to what general conclusions these 
points lead us, if established. 


1. As to external features, we have failed to find in Triton 
the suckers and horny teeth with which the Batrachian 
larva is furnished. 

2. Segmentation proceeds in a manner much like that of 
the Frog, but the roof of the segmentation cavity is from the 
very first only one cell thick. 

3. An unsymmetrical invagination, like that of the Frog 
and Lamprey, takes place, giving rise to one layer in the 
middle line, the hypoblast, and two at the sides, hypoblast 
and mesoblast. The invagination mesoblast is supplemented 
by other cells, which split off from the yolk hypoblast. These 
two lateral and disconnected masses of mesoblast are, we 
consider, the homologues of the paired hypoblastic diver- 
ticula in Amphioxus. 


1 Jenaische, ‘ Zeitschrift,’ 1871, pp. 435-439. 


60 Ww. B. SCOTT AND HENRY F, OSBORN. 


4. The epzblast is at first composed of a single layer of 
columnar cells, which early separate into two rows, and of the 
two layers thus formed the inner becomes the active one, 
entering exclusively into the formation of the sense organs. 
In the spinal cord and brain the division into two layers does 
not take place till very much later. 


5. The hypoblast is of two kinds, the invaginated and that 
which arises from the metamorphosed yolk-cells. 


6. The notochord is of hypoblastic origin, and takes up the 
entire dorsal wall of the alimentary tract (except in the head) 
in its formation, fresh hypoblast growing from the sides 
below it. It becomes well formed and cylindrical in shape 
before any cell division takes place in it. 


7. The body-cavity extends into the head, appearing in 
this region first. The head mesoblast becomes split into 
somites, which have the same relations and number (except 
so far as modified by the reduction of the visceral clefts) as 
in the Elasmobranchs, but do not seem to communicate 
below. 


8. The thyroid body is formed by an outgrowth from the 
alimentary canal, the walls of which become continuous with 
the mucous layer of the epiblast ; the continuity of the horny 
layer is not interrupted. 


Conclusion. 


If the statements in this paper prove to be well founded, 
they will give us some data for judging of the relationships 
of the two groups of Amphibia to each other, and to some 
lower types. The marked divergences from the Batrachian 
type which the Newt shows us point to the conclusion that 
the Urodeles and Batrachians have been separated for a very 
long period. And it is interesting to observe that, in those 
cases where the divergence is other than a mere matter of 
detail, it leads towards the Lamprey, and through that to 
Amphioxus. The opinion seems to be gaining ground that 
some such form as the Lamprey is the point toward which the 
Amphibia, the Elasmobranch, Ganoid, and Dipnoic fishes 
converge, and the more these types are investigated the 
better established appears this view. As yet, however, we 
are not in a position to pronounce upon it with even an 
approximation to certainty. The observations brought 
forward in this paper tend strongly, we think, in this 
direction, and we hope that future investigations upon the 


EARLY DEVELOPMENT OF THE COMMON NEWT. 61 


Amphibia, the Ganoids, and especially the Dipnoi, will soon 
put the matter to a crucial test. 


In conclusion, we must express our very sincere thanks to 
Mr. F. M. Balfour for his never-failing kindness and assist- 
ance to us while engaged in this work. 


Devetorpment of the Kipney in its relation to the 
Wo .rFFian Bopy in the Cutcx. By Apam Sepewiox, 
B.A., Scholar of Trinity College, Cambridge ; 
Demonstrator in the Morphological Laboratory. 
(With Plates VI and VII.) 


THIs paper contains an account of observations on the 
development of the excretory system of the chick, made with 
a view of elucidating the relation which the kidney bears to 
the Wolffian body. 

The Wolffian body in the embryo chick attains to a very 
great development, but almost completely atrophies in the 
adult, a small part only persisting in the male as part of the 
testicular apparatus. 

In the embryos of lower Vertebrates, viz. most of the 
Icthyopsida, there is present, similarly, an organ called the 
Wolffian body, which, however, much more completely 
persists in the adult, functioning in part as kidney and in 
part as semen carrier. 

The separation into an urinary part and into a sexual part 
is much more complete in some forms than in others. In 
the Elasmobranchil, for instance, the posterior part of the 
embryonic Wolffian body gives rise in the adult to a well- 
developed gland, the kidney, while the anterior part attains 
a far less development; in fact, more or less retrogrades in 
the adult ; but in the male a part of it enters into connection 
with the testis. 

In the Amniota the case is different. In them an embryonic 
organ, cailed the Wolffian body, does not function at all in 
the adult as an excretory organ; it almost completely atro- 
phies from its embryonic perfection, only a small part per- 
sisting in the adult male as the epidydimis. The organ 
which functions as kidney in the adult arises at a relatively 
late stage, and is not apparently, as in Elasmobranchs, a 
modified part of the hind end of the embryonic Wolffian 
body. What, then, is the kidney of the Amniota? Is it an 
organ which has arisen de novo in the Amniota, or can it, by 
a more accurate study of its development, be traced into 
relation with the embryonic excretory system? In other 
words, can any evidence be obtained by the study of develop- 
ment proving that the kidney of the chick phylogenetically 


KIDNEY IN RELATION TO WOLFFIAN BODY IN THE CHICK, 683 


has been modified from part of the same primitive organ as 
that from which the Wolffian body developed, as is the case 
in the Icthyopsida ? 

To obtain an answer to these questions I have been 
obliged to make a close study of the earliest stages in the 
development of the kidney and Wolffian body. The results 
obtained with regard to the latter are so different from 
those obtained by the latest observers, that I have recorded 
them in full in the following account. 

Peculiarities in the early development of the Avian 
Woffian body necessitated an examination of the early de- 
velopment of the Wolffian tubules in other Vertebrates. 
This examination I was enabled to make in the case of 
Elasmobranchii owing to the great kindness of Mr. Balfour, 
who placed at my disposal the whole of his Elasmobranch sec- 
tions. The result of this examination was to convince me that 
the account given of the earliest stages in the development 
of the Elasmobranch Wolffian body is in some respects 
erroneous. 

Before proceeding to an account of the observations 
made upon these heads it will be well to give a short 
historical account of the progress of our knowledge on this 
subject, z.e. the development of the Wolffian body and kidney. 

The later views as to the homologies of the parts of the 
excretory system found in the different members of the 
Vertebrate group dates from the work of Balfour! and 
Semper? on the embryology of Elasmobranchs. 

The independent discoveries of these two investigators 
gave an impulse to the study of the development of the 
organs in question in other animals, and as a result it has 
gradually become clearer as the embryology of more animals 
became known that a great similarity in the development of 
these organs characterised the Vertebrata. 

The earlier observers, Remak® and Rathke,* maintained 
that the tubules of the Wolffian body developed indepen- 
dently of the Wolffian duct in a blastema of mesoblast cells 
adjoining the inner side of the duct. 

Waldeyer, in his well-known work,* asserted from his 
observations, that the tubules of the Wolffian body developed 
as outgrowths from the duct, and that the Malpighian bodies 
arose independently in the adjoining mesoblast. ‘The views 


1 * Monograph on the Development of Elasmobranch Fishes.’ 
‘Urogenitalsystem der Plagiostomen. Arbeiten,’ vol. ii. 

‘ Entwickelung der Wirbelthiere,’ &c. 

‘ Entwickelungsgeschichte der Wirbelthiere,’ Leipzig, 1861. 
‘ Kierstock und Hi,’ 1870, 


ao , Ww ho 


64: ADAM SEDGWICE. 


of other observers, before 1874, were identical with one or 
the other of these. 

Since 1874 the work of Gotte! and Spengel? on Am- 
phibia, Kolliker? on Aves, Braun* on Lacertilia, and Fir- 
bringer on Teleostei, Amphibia, and Aves’ has shown that 
the excretory system of all these animals is developed on a 
type seen in its simplest form in Elasmobranchs. 

Kolliker first discovered in Aves structures composed of 
strings of cells connected with the Wolffian duct and peri- 
toneal epithelium, and placed just ventral and internal to the 
former. These he compared to the early segmental tubes 
described in Elasmobranchs. From the similarity of these 
structures to those seen in Elasmobranchs and from his 
own observations he was led to assert for them a deve- 
lopment similar to that described for Elasmobranchs, viz. 
from segmental involutions of the body-cavity epithelium. 

In this he was followed by Furbringer,® except in a small 
detail, the latter observer denying that these cell strings had 
any lumen opening into the body-cavity. 

So far as L know, no ideas as to the morphological 
meaning of the Amniote kidney were held before 1874. 

Conflicting statements were then put forward by different 
observers with regard to the actual embryonic development. 
Remak’ and Kolliker® maintained that the whole of the 
epithelium of the kidney tubules, including that of the 
collecting and secreting tubules and the Malpighian bodies, 
was derived from a simple outgrowth from the ureter. 

The condensed mesoblast tissue which les near the ureter 
and its offshoots, in their opinion, only gives rise to the 
connective and vascular elements of the kidney. 

Kolliker has expressed this view in the second edition of 
his great work on the development of Vertebrates. Lowe? 
has also recently arrived at the same conclusion from his 
observations on Mammals. 


1 ¢ Entwickelungsgesch. d. Unke.’ 

2 “Das Urogen. system d. Amphibien,” ‘Arb. a. d. Zool. Inst.’ 
Wurzburg, Bd. 3, 1876. 

3 * Ent. gesch. d. Menschen u. d. hGheren Thiere.’ 

4 “Das Urogen-system d, d. Hinheimischen Reptilien,’ Semper’s ‘ Ar- 
beiten,’ Bd. 4. 

° “Zur vergleichenden Anat. u. Entwickelungsgeschichte d. Excre- 
tionsorgane der Vertebraten,” ‘ Morphol. Jahrbuch,’ Bd. 4. The reader 
is referred to this admirable essay for the literature, and a complete ac- 
count of our knowledge of the excretory organs of Vertebrates. 

S Asoc. cit. 

7 ‘Entwickelung der Wirbelthiere.’ 

§ Loc. cit. 

® «Centralblatt fiir die Med. Wissenschaften,’ Oct., 1879. 


KIDNEY IN RELATION TO WOLFFIAN BODY IN THE CHICK, 65 


Kupfter,! Bornhaupt,? and Braun,’ on the other hand, 
assert that the secretory tubules and Malpighian bodies are 
formed independently of the ureter in the condensed meso- 
blast tissue mentioned above, the outgrowths from the ureter 
merely giving rise to the collecting tubules. 

I shall return, when I have described the kidney develop- 
ment in the chick, to a consideration and discussion of 
the various hypotheses which have been held concerning the 
Amniote kidney. 

Development of Wolffian body.—The ages of the younger 
embryos from which the sections figured in the accompany- 
ing plates (VI and VII) were taken are indicated by 
the number of protovertebre. In the older embryos this 
was not possible. In most cases the place in the body, 
from which a section figured was taken, is indicated by the 
number of the segment* in which it occurred, counting 
the first segment behind the auditory involutions as the 
first. 

These determinations have been made with some care by 
mounting all the sections in order, and then by observing the 
protovertebre, arranging them into groups corresponding to 
each protovertebra, beginning the process always in front. 

The observations here recorded do not extend to any part 
of the Wolffian body in front of the fourteenth segment, nor 
to the development of the Wolffian duct. I have made 
some observations on both these parts, but they are not yet 
sufficiently complete to enable me to understand certain 
remarkable appearances in their development. The Wolf- 
fian body, like most other organs, develops first of all in 
front and then gradually backwards, so that supposing the 
development behind were the same as in front, the process 
might be shown by a series of sections from a single chick 
of the proper age. But this is not the case. In the chick 
the development of the Wolffian tubules behind is very 
different to that in front. This fact has apparently been 
overlooked by the most recert observers. 

The development of the Wolffian body in the duck is 
much more completely similar throughout than in the chick, 
and reference will be at first made to figs. 2—5, taken from 
a duck embryo with thirty-one or thirty-two protovertebre, 
in the following description. 

' * Arch. f. Mic. Anat.,’ Bd. 1. 

2 < Untersuchungen tiber d. Entwick. des Urogen. systems beim 
Hihnchen,’ Diss. Inaug., Riga, 1867. 

3 Loc. cit. 


‘ The term segment is used as equivalent to protovertebra, muscle 
plate. 


66 ADAM SEDGWICK. 


The tubules of the Wolffian body do not develop from 
serial involutions of the peritoneal epithelium, but from the 
cells of the intermediate cell mass. The intermediate cell 
mass is so familiar to all students of Avian embryology that 


x 


SS 
SoG. 
= 8S 
e 


— 2 Osa, 
Emirs 
ORF 
2. * 
Q 
<s 


Fic. 1.1\—Section from a chick of the second day, showing the inter- 
mediate cell-mass. M.c. Medullary canal. P.v. Protovertebra. | 
W.d. Wolffian duct. p.p. Body-cavity. c.. Notochord. a. o. aorta. 


1 For the woodcuts, figs. 1 and 2, I have to thank Mr. Balfour. Fig. 1 
is from the ‘ Elements of Embryology.’ 


KIDNEY 1N RELATION TO WOLFFIAN BODY IN THE CHICK. 67 


it is hardly necessary to say anything about it. It is a 
mass of cells! (woodcut, fig. 1), stretching between the proto- 
vertebra (P. v.) and the dorsal inner angle of the body- 
cavity (p.p.). It is on its first formation continuous with 
the peritoneal epithelium. Its relation to the protovertebra 
is obscure, and I have been unable, so far, to make it out 
satisfactorily. There is one point, however, to be borne 
in mind concerning this intermediate cell mass; it is con- 
tinuous, 7.¢. is not divided by the lines of segmentation 
into areas corresponding to each protovertebra. 

Very soon after the intermediate cell mass is established 
it undergoes achange. It becomes at some points more dis- 
tinctly continuous with the peritoneal epithelium, at others 
less so. And finally this culminates in a clear continuity, as 
seen in fig. 2, and a marked discontinuity, as seen in fig. 3. 
In fig. 2 we have what practically amounts to a continuation 
of the body-cavity into the intermediate cell mass (2 ¢ m.) ; 
in fig. 3, on the other hand, the intermediate cell mass is 
distinctly disconnected with the peritoneal epithelium, and 
lies as a mass of cells between it and the protovertebra. Al- 
though these figures are not taken from contiguous sections, 
fig. 2 being taken from the thirtieth segment, and fig. 3 from 
the twenty-ninth, yet for all the important details fig. 3 
represents exactly a section next or next but one to fig. 2. 
The intermediate cell mass is then now present as a cord of 
cells continuous from segment to segment, and continuous 
at intervals with the peritoneal epithelium. Fig. 4 repre- 
sents the two conditions of the intermediate cell mass, as 
seen in a single section taken from the twenty-sixth seg- 
ment. 

At the next stage of development the intermediate cell 
mass entirely breaks away from the peritoneal epithelium, 
and lies as a cellular blastema just internal to the Wolffian 
duct. It may be called the Wolffian blastema. 

The Wolffian blastema almost directly breaks up into the 
structures constituting the first rudiments of the Wolffian 
tubules (fig. 5). 

The development of the Wolffian blastema in the chick 
needs further description. : 

In the anterior region of the Wolffian body, as far back as 
the nineteenth or twentieth segments, the above description 
of the conversion of the intermediate cell mass into the 


1 The term intermediate cell mass in this account is only used to in- 
dicate the cell mass connecting a protovertebra with the peritoneal epi- 
thelium, and never refers to the cell mass occupying the same position 
before segmentation has given rise to protovertebre. 


68 ADAM SEDGWICK. 


Wolffian blastema applies (ode fig. 1); but behind this 
region the development is different. 

Here, even before the segments are formed, it is found 
that the cell mass, which on segmentation gives rise to the 
intermediate cell mass, is distinctly separate from the thick 
epithelium of the body-cavity, but attached to the cell mass, 
which will give rise to the protovertebra (fig. 6, taken 
from a chick with twenty-six protovertebre behind the last 
protovertebra). 

And this separation is apparently retained through later 
development. The cell mass (7.¢m’, fig. 6), in the next stage 
(fig. 7, taken from twenty-ninth segment of a chick with 
twenty-nine protovertebre) is obviously the Wolffian blas- 
tema which, in a still later stage (fig. 10, from the twenty- 
ninth segment of a chick with thirty-four), gives rise to the 
commencing Wolffian tubule. 

The same fact may be seen by comparing figs. 8 and 9, 
fig. 8 being taken from the twenty-fourth segment of a chick 
with twenty-six, and fig. 9 from the twenty-fourth segment 
of achick with twenty-nine. 

It will be observed, by inspection of figs. 6, 7, 8, that the 
peritoneal epithelium which adjoins the Wolffian blastema is 
thick, as it is elsewhere; while later (figs. 9, 10) it is in 
the same spot thin, as it is, or will be, in most other parts 
of the body-cavity. 

No doubt this thick epithelium does, in the process of 
becoming thin, bud off cells, which travel inwards, and some 
of which may help to form the definite Wolffian blastema 
(figs. 9,10). But this process takes place everywhere. In 
fact, at an early stage of development almost all the meso- 
blast cells are represented by the thick lining of the body- 
cavity ; and it is by a process of growth inwards of cells from 
this that most of the connective tissue, &c., of the wall of the 
body and gut is derived ; and, therefore, from the analogy of 
the fate of the cells growing out from the body-cavity wall 
in other places, we might fairly assume that those growing 
out from that particular part adjoining the intermediate cell 
mass or Wolffian blastema give rise, not to the Wolffian 
tubules, but to the connective tissue and blood-vessels of the 
Wolffian body. This is rendered highly probable from a 
consideration of some observations I have made on the de- 
velopment of the segmental tubes in Elasmobranchs, which 
point to the conclusion that the epithelial lining is derived 
from the cells of the intermediate cell mass. However that 
may be, of one fact there can be no doubt, viz. the cells from 
which the Wolffian tubules develop are not derived from 


KIDNEY IN RELATION TO WOLLFIAN BODY IN THE CHICK. 69 


serial ingrowths of the body-cavity epithelium, for this thin- 
ning of the peritoneal epithelium, adjoining the region where 
they will appear, is continuous, 2. e. cells must grow in along 
a line extending the whole length of that part of the body- 
cavity which the Wolffian body adjoins. 

The development of the Wolffian blastema, so described, is 
continued as far back as the opening of the Wolffian duct 
into the cloaca, which occurs in the thirty-fourth segment. 
In fig. 12 it may be seen in the thirty-second segment (4). 

The Wolffian blastema of the chick then develops in two 
slightly different ways. 

In the anterior part, about as far back as the twentieth 
(fig. 1) segment, that process of development which has been 
described at length in the case of the duck (in which animal 
it is apparently the only method of development) is passed 
through in the chick. 

Posteriorly from the twentieth segment the intermediate 
cell mass has never any connection with the peritoneal epi- 
thelium, and gives rise to the Wolffian blastema quite inde- 
pendently of the peritoneal epithelium. ‘This latter process 
is clearly an abbreviation of that which takes place through- 
out in the duck and in the anterior part of the chick. 

I have mentioned the twentieth segment as about the limit 
between the two. I cannot fix the exact limit. 

It has been stated above that in the case of the duck and 
the anterior part of the chick (figs. 1, 2, 4) the intermediate 
cell mass becomes, at certain points, very markedly con- 
tinuous with the peritoneal epithelium, and appears to en- 
close a prolongation of the body-cavity (fig. 1 and fig. 2). 
Such connections are undoubtedly rudiments of the nephro- 
stomata seen in other Vertebrates. They do not occur seg- 
mentally, being situated as often as not between the 
protovertebre. 

Rudiments of these rudimentary nephrostomata occur in 
the posterior part of the chick’s Wolffian body; that is, 
although a fairly sharp line can always be drawn between 
the Wolffian blastema and the peritoneal epithelium, yet 
the cells of the latter, at certain points, arrange themselves 
just as they do in front, where the line of the body-cavity is 
continued into the intermediate cell mass. ‘These latter 
rudiments are very obscure, and I have been unable to make 
any satisfactory determination of their number. They may 
be due merely to an accidental arrangement of the cells, 
which might occur in consequence of the bend in the peri- 
toneal epithelium at this point. Whether the rudimentary 
nephrostomata in the anterior part of the chick’s kidney 


70 ADAM SEDGWICK. 


(fig. 11) are continued as small channels in the intermediate 
cell mass or Wolffian blastema, and remain, enlarging when 
the Wolffian tubules are formed later, I have been unable to 
ascertain. Neither have I been able to satisfy myself as to 
another interesting point, viz. do those rudimentary nephro- 
stomata correspond to the Wolffian tubules subsequently 
developed? In the chick’s twentieth segment never more 
than three nephrostomata can, at the most, be made out, 
yet there are four-or five primary Wolffian tubules later. 

Has this increase been caused by the development of more 
tubules than there were nephrosomata, @. e. by intercalation, 
or has it been caused by a change in the relation of the parts 
to one another, due either to an elongation of the proto- 
vertebra or to the travelling forward of the tubules as they 
are developed behind ? 

I shall return to the noneideration = this point in a future 
paper. 

The mode in which the Wolffian blastema behind the 
twentieth? segment breaks up into tubules, so far as I have 
been able to ascertain it, is the following:—A number of 
vesicles, either oval or circular, lined by columnar cells, and 
lying just internal to the Wolffian duct, make their appear- 
ance (fig. 5). In longitudinal sections it may be seen that 
these vesicles closely adjoin one another antero-posteriorly. 
They are developed from those cells of the Wolffian blastema 
immediately adjoining the inner border of the Wolffian duct. 
By a study of transverse sections it appears that each vesicle 
is continuous ventrally with that part of the Wolffian blas- 
tema which has not undergone conversion into the walls of 
the vesicle, and which lies just internal to the vesicle. The 
cells of this part of the Wolffian blastema very soon arrange 
themselves round what appears as a continuation of the 
original vesicle (fig. 11). From the inner and dorsal wall 
of the last-formed structure a glomerulus is ultimately de- 
veloped. The whole structure grows enormously, and gives 
rise to the Malpighian body and complicated coils of the 
jater Wolffian tubule. At about the stage of development 
represented in fig. 11 the tubules acquire an opening into 
the Wolffian duct. 

The question as to whether or no there are outgrowths 
from the Wolffian duct to meet the independently developed 


' Vide also fig. 125, of Kolliker’s ‘ Entwick. gesch. der Menschen u. 
der. h. Thiere.’ 

2 [reserve an account of the development of the tubules in front of the 
twentieth segment, as my observations on this point are not yet sufli- 
ciently complete to enable me to speak with certainty. 


KIDNEY IN RELATION TO WOLFFIAN BODY IN THE CHICK. 71 


Wolffian tubulesis not easy toanswer. I leave it open now, 
but hope to be in a position to give a definite answer soon. 
Now I will merely state that there are appearances in my 
sections which incline me to the opinion that there are out- 
growths from the Wolffian duct which, in the case of the 
primary Wolffian tubules, are solid, but hollow in the case of 
the secondary and tertiary tubules. 

The above description of the development of the primary 
Wolffian tubules differs from the most recent account of 
Kolliker! and Firbringer.” I have stated above the views 
which these distinguished observers hold as to the develop- 
ment. They have described perfectly correctly one stage in 
the development of the anterior part of the Wolffian body. 
I have often seen the appearances given by Kolliker in fig. 
125 of his work, and have given myself a similar represen- 
tation (fig. 1).. But if I understand them correctly they 
have given an erroneous account of the earlier development 
of these structures. Fiirbringer says of them (p. 67): “Sie 
finden sich in reihenweiser Anordnung als solide Urnieren- 
strange die von dem parietalen Peritoneum ventral und 
medial vom Wolff’schen Gange ausgehen. . .. Sehr bald losen 
sich diese Urnierenstrange von dem parietalen Peritoneum 
ab und liegen nun als rundliche solide Zellenmassen retrope- 
ritoneal neben dem Wolff’schen Gange, ein Stadium das die 
Beobachtungen der meisten Autoren deckt.” It is easy to 
see how Fiirbringer has been misled. He has seen in trans- 
verse sections in a fairly young chick the S-shaped strings 
of cells (solide Urnierenstrange) in connection with the peri- 
toneal epithelium. He has also seen in an older chick the 
Wolffian blastema of the posterior region of the Wolffian 
body (rundliche solide Zellenmassen). Both these observa- 
tions I can entirely confirm. But apparently he has not 
examined the condition of these structures at an earlier 
stage, assuming that they originate as solid outgrowths of 
the peritoneal epithelium. This assumption my observa- 
tions prove to be unwarranted. 

The older observers (see above) were quite correct in their 
statements of the origin of the Wolffian tubules, as struc- 
tures developed in the intermediate cell mass, independently 
of the Wolffian duct, and later acquiring an opening into it. 

Waldeyer’s® statement that the Malpighian body thu 
develops, the rest of the Wolffian tubule developing as out- 
growths from the Wolffian duct, is in my opinion erroneous. 
For if there be an outgrowth from the Wolffian duct it 
does not give rise to the whole tubule, exclusive of the 


1 Loc. cit. 2 «Morph. Jahrbuch,’ Bd. 4. 3 Loc cit, 


i2 ADAM SEDGWICK, 


Malpighian body, the structure developed independently 
in the intermediate cell mass certainly giving rise to more 
than the Malpighian body. 

I now pass to the development of the secondary tubules, 
&c. Fiirbringer! derives them also from peritoneal in- 
growths. He has not, however, given, so far as I know, any 
figures showing this. I have examined this point with 
some care, but have quite failed to discover any traces of 
these secondary ingrowths. 

The secondary tubules appear to me to arise from small 
masses of cells, which occupy, at a slightly earlier stage, the 
position of wt. in fig. 11. In this figure the vesicular 
rudiment of a secondary tubule has appeared in this mass of 
cells. The tertiary and quaternary, &c., tubules appear to 
arise successively at a slightly later stage from similar 
small masses of cells, which are always placed just dorsal to 
the last-formed tubule. The later development of these 
secondary, tertiary, &c., Wolffian tubules is very similar to 
that of the primary. 

The time of development of the primary tubules relatively 
to that of the secondary, &c., tubules varies in different parts. 
Anteriorly the primary tubule is much more developed (in 
fact, has acquired an opening into the Wolffian duct), before 
the first trace of the secondary tubule arises, than posteriorly, 
where a secondary and tertiary tubule have appeared almost 
before the primary tubule has lost its vesicular structure 
(fig. 13). 

The development of the secondary, &c., Wolffian tubules in 
the chick appears to be very much abbreviated. 

Whatever may have been their development in phylogeny, 
no light is thrown upon it by their ontogeny. Nor even can 
a comparison be made between their development in the 
chick, and that in other forms in which it is possible to 
suppose the development is less abbreviated. In Elasmo- 
branchs the secondary tubules, as Balfour? has shown, 
develop in connection with the Malpighian bodies of the 
primary tubules, as outgrowths from them, which eventually 
open into the collecting tubules of the segment in front. 
Neither Balfour nor, as far as I know, any other observer, 
have elucidated the development of the tertiary, &c., tubules 
in Elasmobranchs. 

In the Salamander Fiirbringer? has shown that they 
develop as they do in the chick from cell masses closely 


1 Loe. cit. 
2 Balfour, ‘Elasmobranch Fishes.’ 
3 Loc. cit. 


KIDNEY IN RELATION TO WOLFFIAN BODY IN THE CHICK. 73 


adjoining the primary tubules, and from an inspection of his 
figures it 1s evident that these cell masses are situated close to 
the Malpighian body of the primary tubule. In the chick I 
have sought in vain for some clear sign in the development 
of these cells which would enable a comparison to be insti- 
tuted with Elasmobranch development. 

In the chick the cells of the Wolffian blastema do not all 
seem to be used in the formation of the primary tubule. 
‘Those, that are not, seem to collect at a special point, 7. e. 
just dorsal to that part of the primary tubule which will 
become eventually the Malpighian body. The cells of the 
primary tubule are especially thick at this point, and per- 
haps they give rise to some of the cells for the secondary 
tubule (fig. 11). Hvenif this is the case—and we may look 
upon the secondary tubule as an outgrowth from the primary 
tubule—it is impossible to say where the secondary tubule 
so formed opens into the Wolffian duct. 

This brings me to another difference between the dorsal 
tubules of the chick and those of Elasmobranchs. In the latter 
they all open into the collecting part of a primary Wolffian 
tubule. In the former they all open independently into the 
Wolffian duct, or it may be into an outgrowth from it, but 
separately from the primary tubule. This latter point I am, 
as above stated, obliged to leave open at present. The 
number of primary tubules present in one segment seems to 
be fairly constant, five or six to each segment, throughout 
the Wolffian body, except quite in front, where there seem 
to be fewer. All segments, from the twentieth to the 
thirtieth inclusive, contain five or six primary tubules. In 
front of the twentieth segment they seem gradually to de- 
crease. In the first segment in which a fully developed 
tubule appears there seems only to be one, the number 
increasing rapidly to the twentieth. 

The dorsal tubules appear in greater number behind 
than in front. In the twenty-eighth segment I have 
counted as many as four, but more are possibly developed 
later. They correspond in number to the primary tubules, 
t.e. if there are five primary tubules in the twenty-eighth 
segment, there are twenty secondary tubules (five sets of 
four). The most anterior segment in which a secondary 
tubule appears is the twenty-first or twenty-second; I have 
not been able, however, to localise it exactly. 

Development of kidney.—The development of the Wolffian 
biastema from the intermediate cell mass has been described 
as far back as the thirty-fourth segment ; 7. e. to the opening 
of the Wolffian duct into the cloaca; it is never seen 

6 


74 ADAM SEDGWICK. 


behind this point. But it does not all undergo the above 
development into Wolffian tubules. It breaks up into 
Wolffian tubules as far back as the thirtieth segment. Behind 
this point, 2. e. from the thirty-first to the thirty-fourth 
segments inclusively, the Wolffian blastema undergoes quite 
a different fate. It remains for some time almost quite pas- 
sive and ultimately gives rise to the epithelium of the per- 
manent kidneys. In consequence of this I have called that 
part of the Wolffian blastema between the thirty-first and 
thirty-fourth segments the kidney blastema; and in future 
shall refer to it by that name (figs. 12, 15, 16, 17, 46). - It 
is Important to notice that this kidney blastema develops 
in an exactly similar manner to the Wolffian blastema. 

It is not until well into the fourth day, when the ureter 
has appeared, that it is possible to draw the line between 
the two. 

Fig. 12 is taken from the thirty-second segment of a chick 
with thirty-four protovertebre ; it shows a blastema of cells 
lying just internal to the Wolffian duct. Fig. 10 is taken 
from the twenty-ninth segment of the same chick. It shows 
the hindermost trace of a Wolffian tubule I could find at 
this stage. In all the sections between figs. 10 and 12 there 
is present, just as in fig. 12, a blastema of cells lying just 
internal to the Wolffian duct. 

In a slightly older embryo the hindermost trace of a 
Wolffian tubule would be inthe thirtieth segment. In still 
older embryos secondary tubules would have appeared in the 
thirtieth segment, but no trace of a primary tubule in the 
thirty-first, and so on in later stages, Wolffian tubules never 
appearing in the thirty-first segment. In the embryo, from 
which figs. 13 to 17 were taken, the ureter had not appeared. 
In examining a series of sections from the posterior part of 
this embryo, some of which are figured (figs. 13 to 17), the 
following points are noticeable, illustrating what has just 
been stated. 

A primary and secondary tubule are present in fig. 14, and 
it is almost the last section in which any trace of a Wolffian 
tubule can be seen (the two above tubules are cut in the 
next two sections). The tubules adjacent anteriorly to these 
are three in number (fig. 13), consisting of primary, secon- 
dary, and tertiary. Supposing the Wolffian body were going 
on developing in the region behind that from which fig. 14 
was taken, we ought at the least to find at this stage primary 
tubules in that region, for the formation of primary and 
secondary tubules is always separated by an interval of time. 
But no such primary tubules are seen. Behind fig. 14 (figs. 


KIDNEY IN RELATION TO WOLFFIAN BODY IN THE CHICK. 79 


15 to 17) a blastema of cells is still present, precisely 
similar in its appearance and position with regard to the 
Wolffian duct to the Wolffian blastema seen anteriorly at 
eatlier stages, and to the blastema seen in the same region 
at later stages (fig. 12). And this can be traced back to the 
opening of the Wolffian duct into the cloaca (fig. 17). 

To this blastema of cells Ihave given the name kidney 
blastema; it is at this stage perfectly continuous anteriorly 
with the hinder Wolffian tubules, the junction between the 
two lying in one of the two sections intervening between 
figs. 14 and 15. I ought rather to say the line of future 
separation, for so far they have been always continuous, 
having developed so. The continuity between the kidney 
blastema (45) and the hinder part of the Wolffian body 
may be seen in fig. 21, which is taken from a chick of nearly 
the same age as that from which figs. 13—17 were. 

In this figure the section has passed through the hind end 
of the Wolffian duct and through the kidney blastema, and 
has just shaved the hinder end of the Wolffian body, in conse- 
quence of which the hinder Wolffian tubules are only indis- 
tinctly visible. 

The next change to notice is caused by the appearance of 
the ureter. It arises as a growth forward from the dorsal 
border of the enlarged end of the Wolffian duct. This has 
been generally recognised since Kupffer’s! account. The dila- 
tation of the hind end of the Wolffian duct occurs in a very 
slightly later embryo than that from which fig. 21 was taken. 

The kidney blastema is now found not ventrally close to 
the body-cavity, but lies dorsal to its former position, just 
internal to the dorsal extremity of the dilated Wolffian duct 
(fig. 20). From this dilatation there grows forward a duct, 
the ureter (fig. 19),on the inner side of which lies the kidney 
blastema. 

The ureter, at this stage, has not a very great extent, and 
is only seen for a few more sections ; in fig. 18, still behind 
the Wolffian body, the ureter is not visible; but the kidney 
blastema occupies a dorsal position, as it did in the posterior 
section in which the ureter was present (fig. 19). 

In tracing it forward it gradually descends and becomes 
continuous with the hind end of the Wolffian body. 

In yet older embryos, in which the ureter is more developed 
and overlaps the hind end of the Wolffian body, the kidney 
blastema has quite broken off from the Wolffian body, and 
invests the anterior end of the ureter, so that in a series of 
transverse sections through a chick at this age we should see 

1 © Arch. f. M. Anat.,’ Bd, 2. 


76 ADAM SEDGWICK: 


posteriorly, in the same section with the now complicated 
Wolffian body, a dorsal mass of cells. Gradually travelling 
backwards a duct would appear cut across lying in the 
mass of cells; further back still we should see no Wolffian 
body, but merely a duct with a mass of cells lying just | 
internal to it, placed well dorsal to the Wolffian duct. This 
mass of cells is the kidney blastema; and the duct is the ureter. 

Such would be seen in a chick at the end of the fifth day. 

On the sixth day the ureter grows in length, the kidney 
blastema accompanying it, and enveloping its anterior 
extremity. 

The ureter now dilates at intervals, and the kidney blas- 
tema especially collects round these dilatations. From the 
latter, the number of which I have not determined, the 
kidney tubules grow out. In a chick of the seventh day the 
tubules are just beginning to grow out from these dilata- 
tions. The two posterior tubules are, however, far more 
advanced than the anterior. 

The ureter is now a small duct lying just dorsal to the 
Wolffian body ; except at its anterior extremity, where it 
is rather more dorsal, and is completely surrounded by the 
kidney blastema. 

Almost immediately in front of the hind end of the ureter 
a tubule is given off, which runs dorsalwards and outwards. 
The kidney blastema no longer adjoins the ureter, but is 
disposed round the branches of this tubule. The ureter is 
continued forwards through a considerable number of sec- 
tions, giving off no tubules, and unaccompanied by the kidney 
blastema. It then becomes continuous with a tubule, which 
has already been visible in many sections surrounded by 
kidney blastema, and which, though not so much branched 
as the posterior tubule above mentioned, is more developed 
than any tubule met with in front. 

The ureter continues as a small duct lying just dorsal to 
the Wolffian body. 

In this embryo (seventh day), travelling forwards, several 
dilatations could be made out. The appearance presented 
by such a dilatation in transverse section and its position 
with regard to the Wolffian body, may be gathered from an 
inspection of fig. 22. 

In this section the lateral walls of the dorsal part of the 
dilatation of the ureter are closely applied, the lumen being 
very indistinct. 

Around the dorsal part of the dilatation the kidney 
blastema is present. 

In the next section, or in the next section but one (fig. 23), 


KIDNEY IN RELATION TO WOLFFIAN BODY IN THE CHICK, 77 


either backwards or forwards, the dorsal dilatation would be no 
longer visible, but, occupying the position of the dorsal part 
of the dilatation, would be seen a tubule surrounded by the 
cells of the kidney blastema. 

In the next section to this (fig. 24) the walls of the tubule 
become indistinctly marked off from the kidney blastema. 
Some of the large columnar cells of the kidney tubule become 
branched, the processes being continuous with the processes 
of the branched cells of the kidney blastema. In fact, every 
stage of cell shape between a columnar lining cell of the 
tubule and a branched cell of the blastema is visible. 

The lumen of the tubule is no longer distinct, it not being 
possible to say what is an intercellular space and what the 
lumen of the tubule. 

In the next section no trace of a tubule is visible, its 
place being occupied by the cells of the blastema. 

Fig. 23 is taken from a section next but one to fig. 22. 
Nine such dorsal dilatations of the ureter, with commencing 
tubules growing from them, could be made out in the embryo 
under consideration. 

In front of the anterior dilatation the tubules open directly 
into the ureter which in this region has become more dorsal 
with regard to the Wolffian body. Tubules in this region, more- 
over, are given off from the ventral side of the ureter, corre- 
sponding almost exactly to those given off from the dorsal side. 
Four pairs could be made out; after which the ureter ended 
closely surrounded on all sides by dense kidney blastema. 

The next and last stage, which I have closely examined, 
was in an eight-day chick. The kidney had reached a great 
complication of structure. Malpighian bodies had, however, 
not distinctly appeared. The tubules were still surrounded 
by kidney blastema which was especially conspicuous at their 
growing ends. The appearance of the latter, which was ex- 
actly similar in all essential details to the growing points of 
the tubules of the stage last described, is represented in fig. 24. 

Before considering the bearing of the above facts upon the 
questions asked at the outset, I will recapitulate the more 
important points in the development of the Avian kidney 
and Wolffian body. 

1. The cells which give rise to the Wolffian and kidney 
tubules do not develop as involutions of the peritoneal epi- 
thelium, but from a blastema of cells derived from the inter- 
mediate cell mass. 

&. The blastema of the kidney is at first perfectly con- 
tinuous with that of the Wolffian body, and cannot be dis- 
tinguished from it, 


78 ADAM SEDGWICK. 


3. Wolffian tubules do not appear in any part of the blas- 
tema behind the thirtieth segment. Primary, secondary, and 
tertiary, &c., tubules are developed in that part of it placed 
in the thirtieth and anterior segments as far as the twenty- 
first or twenty-second, and primary tubules in yet anterior 
segments. 

4. The blastemain the thirty-first to thirty-fourth segments, 
on the appearance of the ureter, moves dorsalwards from the 
Wolffian duct, breaking away from the hindermost Wolffian 
tubules and enters into close relation with the ureter. 

5. This part of the blastema—the kidney blastema—espe- 
cially collects round swellings on the ureter, from which 
kidney tubules grow out. 

6. These kidney tubules burrow into the kidney blastema, 
their growing points being continuous with the cells of the 
blastema. . 

Five years ago Balfour! and Semper? independently put 
forward the hypothesis that the kidney of the Amniota holds 
the same relation to the embryonic Wolffian body as does the 
adult kidney in Klasmobranchs. 

Balfour wrote then :° ‘‘ The last feature in the anatomy of 
the Selachians which requires notice is the division of the 
kidney into two portions, an anterior and posterior. The 
anatomical similarity between this arrangement and that of 
higher Vertebrates (birds, &c.) is very striking. The anterior 
one precisely corresponds, anatomically, to the Wolfian body, 
and the posterior to the true permanent Avdney of higher 
Vertebrates ; and when we find that in the Selachians the 
duct for the anterior serves also for the semen, as does the 
duct of higher Vertebrates, this similarity seems almost to 
amount to identity.” 

The development of the kidney of the bird has never 
been fully worked out, so that this hypothesis, arrived at from 
a consideration of the facts of comparative anatomy and Elas- 
mobranch embryology, has hitherto not been tested by the 
facts of Avian embryology. The observations described above 
were undertaken with a view of testing this hypothesis, and, 
in my opinion, it has fully stood the test. 

The development of the kidney in the chick points most 
decidedly to the conclusion that it is merely the posterior 
part of the Wolffian body—or, perhaps, it would be better to 
say, of a primitive organ, the anterior part of which is now 


1 “Urogenital Organs of Vertebrates,” ‘Journal of Anatomy and 
Physiology,’ vol. x. 

2 Loe. cit. 

3 P: Pi fe 


4 
2 


~ 


KIDNEY IN RELATION TO WOLFFIAN BODY IN THE CHICK, 19 


seen as the Wolffian body—the whole of this primitive organ 
in Elasmobranch embryos being termed Wolffian body. 

The most important fact in favour of Balfour’s hypothesis 
is the primitive continuity in early stages in the bird of the 
cells from which both Wolffian body and kidney arise. 

The differences in later development cannot be looked 
upon as a serious difficulty when we remember the immense 
differences which many undoubtedly homologous organs 
show in their embryonic development in various animals. 

It has been stated (see above, p. 64) by many students 
of Avian embryology that the kidney tubules develop as 
outgrowths from the ureter, and that the cells of the kidney 
blastema merely give rise to the vascular elements of the 
glomerulus. This view, whether considered @ priori, or with 
reference to the facts of development, cannot for a moment 
be maintained. 

If Balfour’s hypothesis as to the relation of the kid- 
ney to the Wolffian body be accepted, and I do not see 
how it can be rejected, assuming the truth of the facts 
of development recorded in this account, it would re- 
quire very strong proof indeed to establish the fact that 
the cells of the kidney blastema give rise merely to the 
vascular elements of the glomerulus, and take no part in 
the formation of the secretory epithelium of the kidney 
tubules, such as is taken in the formation of tubules of a 
very similar organ by cells developed in a precisely similar 
way. Such proof is not forthcoming, and would be very 
hard to give. 

Considering the very late development of the posterior 
part of the Wolffian body (kidney) in the chick with refer- 
ence to that of the anterior part, it surely cannot bea matter 
of surprise if the development has been modified, the 
walls of the tubule arising from the cells of the blastema ; 
the lumen, however, not as in the anterior part, first appear- 
ing as an independent cavity, which opens later-into the 
duct, but being from the first continuous with the lumen of 
the ureter. 

Firbringer’s suggestion, that the Amniote kidney is de- 
rived from dorsal tubules of the Wolffian body, is based 
mainly on the fact that it lies dorsal to the Wolffian body, 
and an observation of Braun’s for Lizards. Braun has 
stated for these animals that the kidney blastema develops 
from irregular ingrowths of the peritoneal epithelium, ata 
period when the secondary dorsal tubules of the Wolffian 
body are developing. 

With regard to the first point it is to be noted that the 


80 ADAM SEDGWICK, 


dorsal position of the kidney is clearly a secondary change, 
appearing only late in development, and due obviously to 
the great size the kidney attains. Moreover, according to 
Fiirbringer’s view, one would expect to fitid some kind of 
continuity between the developing kidney and dorsal part 
of the Wolffian body; but no trace of any such connection 
can ever be seen. 

Finally, in view of the facts of development here recorded 
for the chick, and of those about to be mentioned for Elas- 
mobranchs, Braun’s observations on the development of the 
kidney blastema of Lizards from peritoneal ingrowths cannot 
be accepted without further evidence. The irregularly 
scattered cells lying between the Wolffian duct and the peri- 
toneal epithelium, which Braun has figuréd, are by no means 
proof of ingrowth of cells from the peritoneal epithelium. 
Such an irregular arrangement of cells can be seen anywhere 
adjoining the body-cavity epithelium. 

Kolhiker’s view that the kidney of the Amniota is an 
organ swt generis, which was not present in any form in the 
excretory system of the common ancestor of Icthyopsida and 
Amniota, needs in my opinion no refutation ; for if true it 
can only be established by proving all other hypotheses con- 
cerning the kidney to be untenable. 

Develupment of segmental tubes in Elasmobranchit.—I 
should hardly have been bold enough to publish these obser- 
vations on the development of the chick’s Wolffian body, 
opposed as they are ‘to statements supported by great 
authority, had I not had the opportunity of examining the 
early development of the parts in question in Elasraobranchs. 

I was thus enabled to confirm: suspicions which I had 
entertained since examining the development of the Wolffian 
body of birds, as to the correctness of the description of the 
earliest stages in these fishes. It is well known that the 
Wolffian tubules of Elasmobranchii are derived from the seg- 
mentally-arranged segmental tubes. ‘These latter were said 
to arise by an invagination, at first solid but subsequently 
becoming hollow, of the peritoneal epithelium just internal 
to the segmental duct into the cells of the intermediate cell 
mass. The intermediate cell mass was said to be produced 
by the coming together of the splanchnic and somatic layers 
of that part of the body cavity, which at an earlier period 
existed connecting the general ventral body cavity with the 
dorsal continuations of it in the muscle plates. 

On examining specimens of young Elasmobranch (Scyl- 
lium, Pristiurus, Torpedo) embryos, I found that the 
passage connecting the general body-cavity with that in the 


KIDNEY IN RELATION TO WOLFFIAN BODY IN THE CHICK, 81 


muscle-plates persisted later than had been described. 
Its connection with the ventral dilatation of the muscle- 
plate cavity is carried ventralwards as far as the outer dorsal 
corner of the segmental duct; so that it appears as a canal 
opening into the body-cavity just internal to the segmental 
duct, and thence curling round its dorsal wall to open into 
the muscle-plate cavity. The ventral outer wall of this 
passage is formed of large columnar cells, the inner and 
dorsal wall of much flatter cells (woodcut, fig. 2), as seen in 
transverse sections. 


Fic, 2.1\—Transverse section through a young embryo of Scyllium. mp. 
Muscele-plate. sf. Segmental tube. sd. Segmental duct. cf. 
Notochord. sp. v, Alimentary canal. 


At the next stage of development the passage becomes 
quite separated from the muscle-plate cavity, and now lies 
as a blind tube, opening into the body-cavity internal to the 
segmental duct, its blind outer end being applied to the 
ventral dilatation of the muscle-plate body-cavity (woodcut, 
fig. 2). This blind tube is the commencement of a segmental 
tube. 

I have traced it through a series of older embryos to the 
fully-formed segmental tube. The adjoining woodcut (fig. 2) 


' Fig. 2 is from Mr, Balfour’s forthcoming treatise on ‘ Comparative 
Embryology.’ It is copied from a figure in his ‘ Monograph on the 
Development of Elasmobranch Fishes,’ 


82 ADAM SEDGWICK. 


represents a section from a Scyllium embryo, in which the 
segmental tube has just broken away from the muscle plate ; 
in a slightly younger embryo, or perhaps in posterior sections 
of the same embryo, the cavity of the segmental tube (s?) 
would communicate with the ventral dilatation of the muscle 
plate (mp), at the point where they are in contact in the 
figure. 

This mode of origin of the segmental tubes of Elasmo- 
branchii renders the origin of the same structures in the 
chick less extraordinary than it at first sight seemed. 

T refrain now from the discussions and perhaps hypotheses 
which this observation on the development of the Elas- 
mobranch segmental tubes suggests. On one point, however, 
there can be little doubt, viz. that segmental involutions of 
the peritoneal epithelium which have been described in the 
Teleostei, Amphibia, Ganoids, Mammals, will have to be 
given up. At any rate the current statement on this point 
cannot be accepted without further proof. 

I have made observations on the Teleostei and Sturgeon 
(Mr.Balfour having kindly put his specimens at my disposal) 
which tend to show, not, however, with certainty, that in the 
embryos of these forms at any rate there are no serial invo- 
lutions of the body-cavity epithelium to form the segmental 
tubes. The method of development in these forms appears 
to me to be very much modified, if we continue to regard the 
development in Elasmobranchs as primitive. 

The Wolffian body in all those animals whose ova have a 
relatively small food yolk seems to be retarded in develop-: 
ment; while the head kidney, the relation of which to the 
rest of the excretory system is not understood, attains an 
early development and functions as the larval excretory 
organ. Very possibly a clue to the explanation of the re- 
tardation in the development of the Wolffian body and of the 
morphelogical meaning of the head kidney, in Teleostei, 
Amphibia, &c., may be obtained by a consideration of this 
coincidence and others hitherto apparently overlooked. 

I hope in another paper to discuss these questions and some 
others which have been passed over here, and to describe the 
development of the anterior part of the Wolffian body in the 
chick. 

In conclusion, I wish to acknowledge the great obligations 
IT am under to Mr. Balfour. Not only have I to thank him 
for his kindness in placing all his preparations and specimens 
at my disposal, but, for what has been far more valuable, 
the help and encouragement he has given me through the 
whole course of this investigation. 


Notes on the Drvenopmunt of the ARANEINA. By 
F. M. Baxrour, M.A., F.R.S., Fellow of Trinity 
College, Cambridge. With Plates VIII, IX, 
and X. 


Tue following observations do not profess to contain a 
complete history of the development even of a single species 
of spider. They are the result of investigations carried on 
at intervals during rather more than two years, on the ova 
of Angeleno labyrinthica ; and I should not have published 
them now, if I had any hope of being able to complete them 
before the appearance of the work I am in the course of 
publishing on Comparative Embryology. It appeared to me, 
however, desirable to publish in full such parts of my 
observations as are completed before the appearance of my 
treatise, since the account of the development of the Aran- 
eina is mainly founded upon them. 

My investigations on the germinal layers and organs have 
been chiefly conducted by means of sections. To prepare 
the embryos for sections, I employed the valuable method 
first made known by Bobretsky. I hardened the embryos 
in bichromate of potash, after placing them for a short time 
in nearly boiling water. They were stained as a whole 
with hematoxylin after the removal of the membranes, and 
embedded for cutting in coagulated albumen. 

The number of inyestigators who have studied the de- 
yelopment of spiders is inconsiderable. A list of them is 
given at the end of the paper. 

The earliest writer on the subject is Herold (No. 4); he 
was followed after a very considerable interval of time by 
Claparéde (No. 3), whose memoir is illustrated by a series 
of beautiful plates, and contains a very satisfactory account 
of the external features of development. 

Balbiani (No. 1) has gone with some detail into the 
history of the early stages; and Ludwig (No. 5) has pub- 
lished some very important observations on the development 
of the blastoderm. Finally, Barrois (No. 2) has quite re- 
cently taken up the study of the group, and has added some 
valuable observations on the development of the germinal 
layers. 

In addition to these papers on the true spiders, important 
inyestigations have been published by Metschnikoff on 
other groups of the Arachnida, notably the scorpion. 
Metschnikoff’s observations on the formation of the ger- 


84. F, M. BALFOUR. 


minal layers and organs accord in most points with my 
own. 

The development of the Araneina may be divided into 
four periods: (1) the segmentation; (2) the period from the 
close of the segmentation up to the period when the segments 
of the body commence to be formed ; (3) the period from the 
commencing formation of the segments to the development of 
the full number of limbs ; (4) the subsequent stages up to 
the attainment of the adult form. 

In my earliest stage the segmentation was already com- 
pleted, and the embryo was formed of a single layer of large 
flattened cells enveloping a central mass of polygonal yolk- 
segments. 

Each yolk-segment is formed of a number of large clear 
somewhat oval yolk-spherules. In hardened specimens the 
yolk-spherules become polygonal, and in ova treated with 
hot water prior to preservation are not unfrequently broken 
up. Amongst the yolk-segments are placed a fair number, 
of nucleated bodies of a very characteristic appearance. 
Each of them is formed of (1) a large, often angular, nucleus, 
filled with deeply staining bodies (nucleoli ?). (2) Ofa layer 
of protoplasm surrounding the nucleus, prolonged into a 
protoplasmic reticulum. The exact relation of these 
nucleated bodies to the yolk-segments is not very easy to 
make out, but the general tendency of my observations is to 
show (1) that each nucleated body belongs to a yolk-sphere, 
and (2) that it is generally placed not at the centre, but to 
one side of a yolk-sphere. If the above conclusions are 
correct each complete yolk-segment is a cell, and each such 
cell consists of a normal nucleus, protoplasm, and yolk- 
spherules. There is a special layer of protoplasm surrounding 
the nucleus, while the remainder of the protoplasm consists 
of a reticulum holding together the yolk-spherules. Yolk- 
cells of this character are seen in Pl. IX and X, figs. 
10—21. 

The nuclei of the yolk-cells are probably derived by divi- 
sion from the nuclei of the segmentation rosettes (v7de Lud- 
wig, No. 5), and it is probable that they take their origin at 
the time when the superficial layer of protoplasm separates 
from the yolk-columns below to form the blastoderm. 

The protoplasm of the yolk-cells undergoes rapid division, 
as is shewn by the fact that there are often two nucleated 
bodies close together, and sometimes two nuclei in a single 
mnass of protoplasm (fig. 10). It is probable that in some 
cases the yolk-spheres divide at the same time as the proto- 
plasm belonging to them; the division of the nucleated 


NOTES UN THE DEVELOPMENT OF THE ARANEINA. 85 


bodies is, however, in the main destined to give rise to fresh 
cells which enter the blastoderm. 

I have not elucidated to my complete satisfaction the next 
stage or two in the development of the embryo; and have 
not succeeded in completely reconciling the results of my 
own observations with those of Claparéde and Balbiani. In 
order to show exactly where my difficulties lie it is necessary 
briefly to state the results arrived at by the above authors. 

According to Claparéde the first differentiation in Pholcus 
consists in the accumulation of the cells over a small area to 
form a protuberance, which he calls the premetive cumulus. 
Owing to its smaller specific gravity the part of the ovum 
with the cumulus always turns upwards, like the blasto- 
dermic pole of a fowl’s egg. 

After a short time the cumulus elongates itself on one 
side, and becomes connected by a streak with a white patch, 
which appears on the surface of the egg, about 90° from the 
cumulus. This patch gradually enlarges, and soon covers 
the whole surface of the ovum except the region where the 
cumulus is placed. It becomes the ventral plate or germinal 
streak of the embryo, its extremity adjoining the cumulus 
is the anal extremity, and its opposite extremity the cephalic 
one. ‘The cumulus itself is placed in a depression on the 
dorsal surface of the ovum. Claparéde compares the cumu- 
lus to the dorsal organ of many Crustacea. 

Balbiani (No. 1) describes the primitive cumulus in 
Tegenaria domestica, Epeira diadema, and Agelena labyrin- 
thica, as originating as a protuberance at the centre of the 
ventral surface, surrounded by a specialised portion of the 
blastoderm (p. 57), which I will call the ventral plate. In 
Tegenaria domestica he finds that it encloses the so-called 
yolk-nucleus, p. 62. By an unequal growth of the ventral 
plate the primitive cumulus comes to be placed at the 
cephalic pole of the ventral plate. The cumulus now 
becomes less prominent, and in a few cases disappears. In 
the next stage the central part of the ventral plate becomes 
very prominent and forms the procephalic lobe, close to the 
anterior border of which is usually placed the primitive 
cumulus (p. 67). The space between the cumulus and the 
procephalic lobe grows larger, so that the latter gradually 
travels towards the dorsal surface and finally vanishes. 
Behind the procephalic lobe the first traces of the segments 
make their appearance, as three transverse bands, before a 
distinct anal lobe becomes apparent. 

The points which require to be cleared up are, (1) what 
is the nature of the primitive cumulus? (2) where is it 


86 F. M. BALFOUR. 


situated in relation to the embryo? Before attempting to 
answer these questions I will shortly describe the develop- 
ment, so far as I have made it out, for the stages during 
which the cumulus is visible. 

The first change that I find in the embryo (when examined 
after it has been hardened)! is the appearance of a small, 
whitish spot, which is at first very indistinct. A section 
through such an ovum (Pl. IX, fig. 10) shows that the cells 
of about one half of the ovum have become more columnar 
than those of the other half, and that there is a point (pr. ce.) 
near one end of the thickened half where the cells are more 
columnar, and about two layers or so deep. It appears tome 
probable that this point is the whitish spot visible in the hard- 
ened ovum. In a somewhat later stage (Pl. VIII, fig. 1) the 
whitish spot becomes more conspicuous (pc), and appears as 
a distinct prominence, which is, without doubt, the primitive 
cumulus, and from it there proceeds on one side a whitish 
streak. The prominence,as noticed by Claparéde and Balbiani, 
is situated on the flatter side of the ovum. Sections at this 
stage show the same features as the previous stage, except 
that (1) the cells throughout are smaller, (2) those of the 
thickened hemisphere of the ovum more columnar, and (8) 
cumulus is formed of several rows of cells, though not 
divided into distinct layers. In the next stage the appear- 
ances from the surface are rather more obscure, and in some 
of my best specimens a coagulum, derived from the fluid 
surrounding the ovum, covers the most important part of the 
blastoderm. In Pl. VIII, fig. 2, I have attempted to repre- 
sent, as truly as I could, the appearances presented by the 
ovum. There is a well-marked whitish side of the ovum, 
near one end of which is a prominence (pc), which must, 
no doubt, be identified with the cumulus of the earlier stages. 
Towards the opposite end, or perhaps rather nearer the 
centre of the white side of the ovum, is an imperfectly 
marked triangular white area. There can be no doubt that 
the line connecting the cumulus with the triangular area is 
the future long axis of the embryo, and the white area is, 
without doubt, the procephalic lobe of Balbiani. 

A section of the ovum at this stage is represented in 
Pl. IX, fig. 11. It is not quite certain in what direction 
the section is taken, but very probably it is somewhat 
oblique to the long axis. However this may be, the section 

1 I was unfortunately too much engaged, at the time when the eggs 
were collected, to study them in the fresh condition; a fact which has 


added not a little to my difficulties in elucidating the obscure points in 
the early stages. 


NOTES ON THE DEVELOPMENT OF THE ARANEINA. 87 


shows that the whitish hemisphere of the blastoderm is 
formed of columnar cells, for the most ‘part two or so 
layers deep, but that there is, not very far from the middle 
line, a wedge-shaped internal thickening of the blastoderm 
where the cells are several rows deep. With what part 
visible in surface view this thickened portion corresponds is 
not clear. To my mind it most probably corresponds to the 
larger white patch, in which case I have not got a section 
through the terminal prominence. In the other sections of 
the same embryo the wedge-shaped thickening was not so 
marked, but it, nevertheless, extended through all the sections. 
It appears to me very possible that it constitutes a longitu- 
dinal thickened ridge of the blastoderm. In any case it is clear 
that the white hemisphere of the blastoderm is a thickened 
portion of the blastoderm, and that the thickening is in part 
due to the cells being more columnar, and, in part, to their 
being more than one row deep, though they have not become 
divided into two distinct germinal layers. It is further clear 
that the increase in the number of cells in the thickened 
part of the blastoderm is, im the main, a result of the mul- 
tiplication of the original single row of cells, while a careful 
examination of my sections proves that it is also partly due 
to cells, derived from the yolk, having been added to the blas- 
toderm. 

In the following stage which I have obtained (which 
cannot be very much older than the previous stage, because 
my specimens of it come from the same batch of eggs), a 
distinct and fairly circumscribed thickening forming the 
ventral surface of the embryo has become established. Though 
its component parts are somewhat indistinct, it appears to 
consist of a procephalic lobe, a less prominent caudal lobe, 
and an intermediate portion divided into about three seg- 
ments; butits constituents cannot be certainly identified with 
* the structures visible in the previous stage. I am inclined, 
however, to identify the anterior thickened area of the pre- 
vious stage with the procephalic lobe, and a slight protu- 
berance of the caudal portion (visible from the surface) with 
the primitive cumulus. I have, however, failed to meet 
with any trace of the cumulus in my sections. 

To this stage, which forms the first of the second period 
of the larval history, I shall return, but it is necessary 
now to go back to the observations of Claparéde and Bal- 
biani. 

There can, in the first place, be but little doubt that what 
I have called the primitive cumulus in my description is the 
structure so named by Claparéde and Balbiani. 


88 ¥. M. BALFOUR, 


It is clear that Balbiani and Claparéde have both failed to 
appreciate the importance of this organ, which my observa- 
tions show to be the part of the ventral thickening of the 
blastoderm where two rows of cells are first established, and 
therefore the point where the first traces of the future meso- 
blast become visible. 

Though Claparéde and Balbiani differ somewhat as to the 
position of the organ, they both make it Jast longer than I 
do: I feel certainly inclined to doubt whether Claparéde is 
right in considering a body he figures after six segments are 
present, to be the same as the dorsal organ of the embryo 
before the formation of any segments, especially as all the 
stages between the two appear to have escaped him. In 
Agelena there is undoubtedly no organ in the position he 
gives when six segments are formed. 

Balbiani’s observations accord fairly with my own up to 
the stage represented in fig. 2. Beyond this stage my own 
observations are not satisfactory, but I must state that I feel 
doubtful whether Balbiani is correct in his description of the 
gradual separation of the procephalic lobe and the cumulus, 
and the passage of the latter to the dorsal surface, and 
think it possible that he may have made a mistake as to 
which side of the procephalic lobe, in relation to the parts 
of the embryo, the cumulus is placed. 

Although there appear to be grounds for doubting whether 
either Balbiani and Claparéde are correct in the position 
they assign to the cumulus, my observations scarcely warrant 
me in being very definite in my statements on this head, but, 
as already mentioned, I am inclined to place the organ near 
the posterior end (and therefore, as will be afterwards shown, 
in a somewhat dorsal situation) of the ventral embryonic 
thickening. 

In my earliest stage of the third period there is present, 
as has already been stated, a procephalic lobe, and an indis- 
tinct aud not very prominent caudal portion, and about 
three segments between the two. The definition of the 
parts of the blastoderm at this stage is still very imperfect, 
but from subsequent stages it appears to me probable that the 
first of the three segments is that of the first pair of ambu- 
latory limbs, and that the segments of the chelicerz and pedi- 
palpi are formed later than those of the first three ambula- 
tory appendages. 

Balbiani believes that the segment of chelicerz is formed 
later than the segments immediately behind it. He further 
concludes, from the fact that this segment is cut off from the 
procephalie portion in front, that it is really part of the 


NOTES ON THE DEVELOPMENT OF THE ARANEINA, 89 


procephalic lobe. I cannot accept the validity of this argu- 
ment; though I am glad to find myself in, at any rate, 
partial harmony with the distinguished French embryologist 
as to the facts. Balbiani denies for this stage the existence 
of a caudal lobe. There is certainly, as is very well shown 
in my longitudinal sections, a thickening of the blastoderm in 
the caudal region, though it is not so prominent in surface 
views as the procephalic lobe. 

A transverse section through an embryo at this stage (PI. 
IX, fig. 12) shows that there is a ventral plate of somewhat 
columnar cells more than one row deep, and a dorsal portion 
of the blastoderm formed of a single row of flattened cells. 
Every section at this stage shows that the inner layer of 
cells of the ventral plate is receiving accessions of cells from 
the yolk, which has not to any appreciable extent altered 
its constitution. A large cell, passing from the yolk to the 
blastoderm, is shown in fig. 12 at y. ¢. 

The cells of the ventral plate are now divided into two 
distmet layers. The outer of these is the epiblast, the 
inner the mesoblast. The cells of both layers are quite 
continuous across the median line, and exhibit no trace of a 
bilateral arrangement. 

This stage is an interesting one on account of the striking 
similarity which (apart from the amnion) exists between a 
section through the blastoderm of a spider and that of an 
insect immediately after the formation of the mesoblast. 
The reader should compare Kowalevsky’s (‘Mem. Acad. 
Petersbourg,’ vol. xvi, 1871) fig. 26, pl. ix with my fig. 
12. The existence of a continuous ventral plate of meso- 
blast has been noticed by Barrois (p. 532), who states that 
the two mesoblastic bands originate from the longitudinal 
division of a primitive single band. 

In a slightly later stage (Pl. VIII, fig. 3 a@ and 33) six 
distinct segments are interpellated between the procephalic 
and the caudal lobes. The two foremost, ch and pd (especially 
the first), of these are far less distinct than the remainder, 
and the first segment is very indistinctly separated from 
the procephalic lobe. From the indistinctness of the 
first two somites, I conclude that they are later formations 
than the four succeeding ones. The caudal and procephalic 
lobes are very similar in appearance, but the procephalic lobe 
is slightly the wider of the two. There is a slight protuber- 
ance on the caudal lobe, which is possibly the remnant of 
the cumulus. The superficial appearance of segmentation 
is produced by a series of transverse valleys, separating 
raised intermediate portions which form the segments. The 


7 


90 F. M. BALFOUR. 


ventral thickening of the embryo now occupies rather more 
than half the circumference of the ovum. 

Transverse sections show that considerable changes have 
been effected in the constitution of the blastoderm. In the 
previous stage, the ventral plate was formed of an uniform 
external layer of epiblast, and a continuous internal layer 
of mesoblast. The mesoblast has now become divided along 
the whole length of the embryo, except, perhaps, the proce- 
phalic lobes, into two lateral bands which are not continuous 
across the middle line (Pl. IX, fig. 13 me). It has, more- 
over, become a much more definite layer, closely attached 
to the epiblast. Between each mesoblastic band and the 
adjoining yolk there are placed a few scattered cells, which 
in a somewhat later stage become the splanchnic mesoblast. 
These cells are derived from the yolk-cells; and almost 
every section contains examples of such cells in the act of 
joining the mesoblast.. 

The epiblast of the ventral plate has not, to any great 
extent, altered in constitution. It is, perhaps, a shade 
thinner in the median line than it is laterally. The division 
of the mesoblast plate into two bands, together, perhaps, 
with the slight reduction of the epiblast in the median 
ventral line, gives rise at this stage to an imperfectly 
marked median groove. 

The dorsal epiblast is still formed of a single layer of flat 
cells. In the neighbourhood of this layer the yolk nuclei 
are especially concentrated. The yolk itself remains as 
before. 

The segments continue to increase regularly, each fresh 
segment being added in the usual way between the last 
formed segment and the unsegmented caudal lobe. At the 
stage when about nine or ten segments have become estab- 
lished, the first rudiments of appendages become visible. 
At this period (Pl. VIII, fig. 4) there is a distinct median 
ventral groove, extending through the whole length of the 
embryo, which becomes, however, considerably shallower 
behind. The procephalic region is distinctly bilobed. The 
first segment (that of the chelicere) is better marked off 
from it than in the previous stage, but is without a trace 
of an appendage, and exhibits therefore, in respect to the 
development of its appendages, the same retardation that 
characterised its first appearance. The next five segments, 
viz. those of the pedipalpi and four ambulatory appendages, 
present a very well-marked swelling at each extremity. 
These swellings are the earliest traces of the appendages. 
Of the three succeeding segments, only the first is well 


NOTES ON THE DEVELOPMENT OF THE ARANEINA, 91 


differentiated. The caudal lobe, though less broad than the 
procephalic lobe, is still a widish structure. The most 
important internal changes concern the mesoblast, which is 
now imperfectly though distinctly divided into somites, 
corresponding with segments visible externally. Each meso- 
blastic somite is formed of a distinct somatic layer closely 
attached to the epiblast, and a thinner and less well-marked 
splanchnic layer. In the appendage-bearing segments the 
somatic layer is continued up into the appendages. 

The epiblast is distinctly thinner in the median line than 
at the two sides. 

The next stage figured (Pl. VIII, figs. 5 and 6) is an im- 
portant one, as it is characterised by the establishment of 
the full number of appendages. The whole length of the 
ventral plate has greatly increased, so that it embraces 
nearly the circumference of the ovum, and there is left 
uncovered but a very small arc between the two extremities 
of the plate (Pl. VIII, fig. 6; Pl. IX, fig. 15). This are 
is the future dorsal portion of the embryo, which lags in its 
development immensely behind the ventral portion. 

There isa very distinctly bilobed procephalic region (pr. /) 
well separated from the segment with the chelicere (ch). It 
is marked by a shallow groove opening behind into a circular 
depression (st.)—the earliest rudiment of the stomodzeum. 
The six segments behind the procephalic lobes are the six 
largest, and each of them bears two prominent appendages. 
They constitute the six appendage-bearing segments of the 
adult. The four future ambulatory appendages are equal in 
size: they are slightly larger than the pedipalpi, and these 
again than the cheliceree. Behind the six somites with pro- 
minent appendages there are four well-marked somites, each 
with a small protuberance. These four protuberances are 
provisional appendages. They have been found in many 
other genera of Araneina (Claparéde, Barrois). The segments 
behind these are rudimentary and difficult to count, but 
there are, at any rate, five, and at a slightly later stage 
probably six, including the.anal lobe. These fresh segments 
have been formed by the continued segmentation of the anal 
lobe, which has greatly altered its shape in the process. The 
ventral groove of the earlier stage is still continued along 
the whole length of the ventral plate. 

By the close of this stage the full number of post-cephalic 
segments has become established. They are best seen in the 
longitudinal section (Pl. IX, fig. 15). There are six anterior 
appendage-bearing segments, followed by four with rudimen- 
tary appendages (not seen in this figure), and six without 


92 F, M. BALFOUR. 


appendages behind. There are, therefore, sixteen in all. 
This number accords with the result arrived at by Barrois, 
but is higher by two than that given by Claparéde. 

The germinal layers (vzde Pl. IX, fig. 14) have by this 
stage undergone a further development. The mesoblastic 
somites are more fully developed. The general relations of 
these somites 1s shown in longitudinal section in Pl. IX, 
fig. 15, and in transverse section in Pl. IX, fig. 14. Inthe 
tail, where they are simplest (shown on the upper side in fig. 
14), each mesoblastic somite is formed of a somatic layer of 
more or less cubical cells attached to the epiblast, and a 
splanchnic layer of flattened cells. Between the two is placed a 
completely circumscribed cavity, which constitutes part of the 
embryonic body cavity. Between the yolk and the splanchnic 
layer are placed a few scattered cells, which form the latest 
derivatives of the yolk-cells, and are to be reckoned as part of 
the splanchnic mesoblast. The mesoblastic somites do not 
extend outwards beyond the edge of the ventral plate, and 
the corresponding mesoblastic somites of the two sides do 
not nearly meet in the middle line. In the limb-bearing 
somites the mesoblast has the same general characters as in 
the posterior somites, but the somatzc layer is prolonged as 
a hollow papilliform process into the limb, so that each limb 
has an axial cavity continuous with the section of the body 
cavity of its somite. The description given by Metschnikoff 
of the formation of the mesoblastic somites in the scorpion, 
and their continuation into the limbs, closely corresponds 
with the history of these parts in spiders. In the region of 
each procephalic lobe the mesoblast is present as a continuous 
layer underneath the epiblast, but in the earlier part of the 
stage, atany rate, is not formed of two distinct layers with 
a cavity between them. 

The epiblast at this stage has also undergone important 
changes. Along the median ventral groove it has become 
very thin. On each side of this groove it exhibits in each 
appendage-bearing somite a well-marked thickening, which 
gives in surface views the appearance of a slightly raised 
area (Pl. VIII, fig. 5), between each appendage and the 
median line. These thickenings are the first rudiments of 
the ventral nerve ganglia. The ventral nervecord at this stage 
is thus formed of two ridge-like thickenings of the epiblast, 
widely separated in the median line, each of which is con- 
stituted of a series of raised divisions—the ganglia (fig. 14, xg) 
—united by shorter, less prominent divisions. The nerve 
cords are formed from before backwards, and are not at this 
stage found in the hinder segments. There is a distinct 


NOTES ON THE DEVELOPMENT OF THE ARANEINA. 93 


ganglionic thickening for the chelicere quite independent of 
the procephalic lobes. 

In the procephalic lobes the epiblast is much thickened, 
and is formed of several rows of cells. The greater part of 
it is destined to give rise to the supra-wsophageal ganglia. 

During the various changes which have been described the 
blastoderm cells have been continually dividing, and, together 
with their nuclei, have become considerably smaller than at 
first. The yolk cells have in the meantime remained much 
as before, and are, therefore, considerably larger than the 
nuclei of the blastoderm cells. They are more numerous 
than in the earlier stages, but are still surrounded by a 
protoplasmic body, which is continued into a protoplasmic 
reticulum. ‘The yolk is still divided up into polygonal seg- 
ments, but from sections it would appear that the nuclei are 
more numerous than the segments, though I have failed to 
arrive at quite definite conclusions on this point. 

As development proceeds the appendages grow longer and 
gradually bend inwards. They become very soon divided by 
a series of ring-like constrictions which constitute the first 
indications of the future joints (Pl. VII', fig. 6). The full 
number of joints are not at once reached, but in the ambu- 
latory appendages five only appear at first to be formed. 
There are four joints in the pedipalpi, while the chelicerz do 
not exhibit any signs of becoming jointed till somewhat later. 
The primitive presence of only five joints in the ambulatory 
appendages is interesting, as this number is permanent in 
Insects and in Peripatus. 

The next stage figured forms the last of the third 
period (Pl. VIII, fig. 7 and 7a). The ventral plate 
is still rolled round the egg (fig. 7), and the end of the tail 
and the procephalic lobes nearly meet dorsally, so that there 
is but a very slight development of the dorsal region. There 
are the same number of segments as before, and the chief 
differences in appearance between the present and the 
previous stage depend upon the fact (1) that the median 
ventral integument between the nerve ganglia has become 
wider, and at the same time thinner ; (2) that the limbs have 
become much more developed; (3) that the stomodzum is 
definitely established ; (4) that the procephalic lobes have 
undergone considerable development. 

Of these features, the three last require a fuller descrip- 
tion. The limbs of the two sides are directed towards each 
other, and nearly meet in the ventral line. The chelicerze 
are two-jointed, and terminate in what appear like rudi- 
mentary chele, a fact which perhaps indicates that the 


94, F, M. BALFOUR. 


spiders are descended from ancestors with chelate chelicere. 
The four embryonic post-ambulatory appendages are now at 
the height of their development. 

The stomodeum (Pl. VIII, fig. 7, and Pl. IX, fig. 17, s¢) 
is a deepish pit between the two procephalic lobes, and dis- 
tinctly in front of segment of the chelicere. It is bordered 
in front by.a large, well-marked, bilobed upper lip, and be- 
hind by a smaller lower lip. The large upper lip is a tem- 
porary structure, to be compared, perhaps, with the gigantic 
upper lip of the embryo of Chelifer (cf. Metschinkoff). On 
each side of and behind the mouth two whitish masses are 
visible, which are the epiblastic thickenings which constitute 
the ganglia of the chelicere (Pl. VIII, fig. 7, ch. 9). 

The procephalic lobes (pr. 7) now form two distinct 
masses, and each of them is marked by a semicircular groove, 
dividing it into a narrower anterior and a broader posterior 
division. 

In the region of the trunk the general arrangement of the 
germinal layers has not altered to any great extent. The 
ventral ganglionic thickenings are now developed in all the 
segments in the abdominal as well as in the thoracic region. 
The individual thickenings themselves, though much more 
conspicuous than in the previous stage (Pl. IX, fig. 16, v.c), 
are still integral parts of the epiblast. They are more widely 
separated than before in the middle line. The mesoblastic 
somites retain their earlier constitution (Pl. IX, fig. 16). 
Beneath the procephalic lebes the mesoblast has, in most 
respects, a constitution similar to that of a mesoblastic somite 
in the trunk. It is formed of two bodies, one on each side, 
each composed of a splanchnic and somatic layer (Pl. IX, 
fig. 17, sp. and so), enclosing between them a section of the 
body-cavity. But the cephalic somites, unlike those of the 
trunk, are united by a median bridge of mesoblast, in which 
no division into two layers can be detected. This bridge 
assists in forming a thick investment of mesoblast round the 
stomodeum (s?Z). 

The existence of a section of the body cavity in the 
preoral region is a fact of some interest, especially when 
taken in connection with the discovery, by Kleinenberg, of a 
similar structure in the head of Lumbricus. The procephalic 
lobe represents the przoral lobe of Chzetopod larvz, but the 
prolongation of the body cavity into it does not, in my opinion, 
necessarily imply that it is equivalent to a post-oral segment. 

The epiblast of the procephalic lobes is a thick layer 
several cells deep, but without any trace of a separation of 
the ganglionic portion from the epidermis. 


NOTES ON THE DEVELOPMENT OF THE ARANEINA. 95 


The nuclei of the yolk have increased in number, but the 
yolk, in other respects, retains its earlier characters. 

The next period in the development is that in which the 
body of the embryo gradually acquires the adult form. The 
most important event which takes place during this period 
is the development of the dorsal region of the embryo, which, 
up to its commencement, is practically non-existent. Asa 
consequence of the development of the dorsal region, the 
embryo, which has hitherto had what may be called a dorsal 
flexure, gradually unrolls itself, and acquires a ventral 
flexure. This change in the flexure of the embryo is in 
appearance a rather complicated phenomenon, and has been 
somewhat differently described by the two naturalists who 
have studied it in recent times. 

For Claparéde the prime cause of the change of flexure 
is the translation dorsalwards of the limbs. He compares 
the dorsal region of the embryo to the arc ofa circle, the 
two ends of which are united by a cord formed by the line 
of insertion of the hmbs. He points out that if you bring 
the middle of the cord, so stretched between the two ends of 
the arc, nearer to the summit of the arc, you necessarily 
cause the two ends of the arch to approach each other, or, in 
other words, if the insertion of the limbs is drawn up dor- 
sally, the head and tail must approach each other 
ventrally. 

Barrois takes quite a different view to that of Claparéde, 
which will perhaps be best understood if I quote a transla- 
tion of his own words. He says: ‘At the period of the 
last stage of the embryonic band (the stage represented in 
Pl. IX, fig. 7, in the present paper) this latter completely 
encircles the egg, and its posterior extremity nearly ap- 
proaches the cephalic region. Finally, the germinal bands, 
where they unite at the anal lobe (placed above on the 
dorsal surface), form between them a very acute angle. 
During the following stages one observes the anal segment 
separate further and further from the cephalic region, and 
approach nearer and nearer to the ventral region. This dis- 
placement of the anal segment determines, in its turn, a 
modification in the divergence of the anal bands; the angle 
which they form at their junction tends to become more 
obtuse. The same processes continue regularly till the anal 
segment comes to occupy the opposite extremity to the 
cephalic region, a period at which the two germinal bands 
are placed in the same plane and the two sides of the obtuse 
angle end by meeting in a straight line. If we suppose a 
continuation of the same phenomenon it is clear that the 


06 F. M, BALFOUR. 


anal segment will come to occupy a position on the ventral 
surface, and the germinal bands to approach, but in the 
inverse way, so as to form an angle opposite to that which 
they formed at first. This condition ends the process by 
which the posterior extremity of the embryonic band, at first 
directed towards the dorsal side, comes to bend in towards 
the ventral region.” 

Neither of the above explanations is to my mind perfectly 
satisfactory. The whole phenomenon appears to me to be 
very simple, and to be caused by the elongation of the 
dorsal region, 7.e. the region on the dorsal surface between 
the anal and procephalic lobes. Such an elongation neces- 
sarily separates the. anal and procephalic lobes; but, since 
the ventral plate does not become shortened in the process, 
and the embryo cannot straighten itself on account of the 
egg-shell, it necessarily becomes flexed, and such flexure can 
only be what I have already called a ventral flexure. If there 
were but little food yolk this flexure would cause the whole 
embryo to be bent in, so as to have the ventral surface con- 
cave, but instead of this the flexure is confined at first to the 
two bands which form the ventral plate. These bands are 
bent in the natural way (PI. VIII, fig. 8, 8), but the yolk 
forms a projection,a kind of yolk sack as Barrois calls it, dis- 
tending the thin integument between the two ventral bands. 
This yolk sack is shown in surface view in Pl. VIII, fig. 8, 
and in section in Pl. X, fig. 18. At a later period, when 
the yolk has become largely absorbed in the formation of 
various organs, the true nature of the ventral flexure 
becomes apparent, and the abdomen of the young Spider is 
found to be bent over so as to press against the ventral 
surface of the thorax (Pl. VIII, fig. 9). This flexure is 
shown in section in Pl. X, fig. 21. 

At the earliest stage of this period of which I have ex- 
amples, the dorsal region has somewhat increased, though 
not very much. The limbs have grown very considerably and 
now cross in the middle line. 

The ventral ganglia, though not the supra-cesophageal, 
have become separated from the epiblast. 2 

The yolk nuclei, each surrounded by protoplasm as before, 
are much more numerous. 

In other respects there are no great changes in the internal 
features. 

In my next stage, represented in Pl. VIII, figs. 8 a, and 
8 b,a very considerable advance has become effected. In 
the first place the dorsal surface has increased in length to 
rather more than one half the circumference of the ovum. 


~ 


NOTES ON THE DEVELOPMENT OF THE ARANEINA, 99 


The dorsal region has, however, not only increased in length, 
but also in definiteness, and a series of transverse markings 
(fig. 8 a and 0), which are very conspicuous in the case of 
the four anterior abdominal segments (the segments with 
rudimentary appendages), have appeared, indicating the 
limits of segments dorsally. The terga of the somites may, 
in fact, be said to have become formed. The posterior terga 
(fig. 8 @) are very narrow compared to the anterior. 

The caudal protuberance is more prominent than it was, 
and somewhat bilobed ; it is continued on each side into 
one of the bands, into which the ventral plate is divided. 
These bands, as is best seen in side view (fig. 8 4), 
have a ventral curvature, or, perhaps more correctly, are 
formed of two parts, which meet at a large angle open to- 
wards the ventral surface. ‘The posterior of these parts bears 
the four still very conspicuous provisional appendages, and 
the anterior the six pairs of thoracic appendages. The four 
ambulatory appendages are now seven-jointed, as in the 
adult, but though longer than in the previous stage they 
do not any longer cross or even meet in the middle line, but 
are, on the contrary, separated by a very considerable in- 
terval. This is due to the great distension by the yolk of 
the ventral part of the body, in the interval between the 
two parts of the original ventral plate. The amount of this 
yolk may be gathered from the section (Pl. X, fig. 18). 
The pedipalpi carry a blade on their basal joint. The che- 
licerze no longer appear to spring from an independent post- 
oral segment. 

There is a conspicuous lower lip, but the upper lip is less 
prominent than before. Sections at this stage show that 
the internal changes have been nearly as considerable as the 
external. 

The dorsal region is now formed of a (1) flattened layer of 
epiblast cells, and a (2) fairly thick layer of large and rather 
characteristic cells which any one who has studied sections 
of spider’s embryos will recognise as derivatives of the yolk. 
These cells are not, therefore, derived from prolongations 
of the somatic and splanchnic layers of the already formed 
somites, but are new formations derived from the yolk. They 
commenced to be formed at a much earlier period, and some 
of them are shown in the long section (Pl. IX, fig. 15). 
In the next stage these cells become differentiated into the 
somatic and splanchnic mesoblast layers of the dorsal region 
of the embryo. 

In the dorsal region of the abdomen the heart has already 
become established. So far as I have been able to make 


98 F. M. BALFOUR. 


out it is formed from a solid cord of the cells of the dorsal 
region. The peripheral layer of this cord gives rise to the 
walls of the heart, while the central cells become converted 
into the corpuscles of the blood. 

The rudiment of the heart is in contact with the epiblast 
above, and there is no greater evidence of its being derived 
from the splanchnic than from the somatic mesoblast ; it is, 
in fact, formed before the dorsal mesoblast has become dif- 
ferentiated into two layers. 

In the abdomen three or four transverse septa, derived 
from the splanchnic mesoblast, grow a short way into the 
yolk. They become more conspicuous during the succeed- 
ing stage, and are spoken of in detail in the description of 
that stage. In the anterior part of the thorax a longitu- 
dinal and vertical septum is formed, which grows downwards 
from the median dorsal line, and divides the yolk in this 
region into two parts. In this septum there is formed at a 
later stage a vertical muscle attached to the suctorial part 
of the stomodeum. 

The mesoblastic somites of the earlier stage are but little 
modified ; and there are still prolongations of the body 
cavity into the limbs (PI. X, fig. 18). 

The lateral parts of the ventral nerve cords are now at 
their maximum of separation (Pl. X, fig. 18, » g). 
Considerable differentiation has already set in in the con- 
stitution of the ganglia themselves, which are composed of 
an outer mass of ganglion cells enclosing a kernel of nerve 
fibres, which lie on the inner side and connect the successive 
ganglia. There are still distinct thoracic and abdominal 
ganglia for each segment, and there is also a pair of separate 
ganglion for the chelicerz, which assists, however, in forming 
the ceesophageal commissures. 

The thickenings of the preeoral lobe which form the supra- 
cesophageal ganglia are nearly though not quite separated from 
the epiblast. The semicircular grooves of the earlier stages 
are now deeper than before, and are well shown in sections 
nearly parallel to the outer anterior surface of the ganglion 
(Pl. X, fig. 19). The supra-cesophageal ganglia are still 
entirely formed of undifferentiated cells, and are without 
commissural tissue like that present in the ventral ganglia. 

The stomodzeum has considerably increased in length, and 
the proctodeum has become formed as a short, posteriorly 
directed involution of the epiblast. I have seen traces of 
what I believe to be two outgrowths from it, which form 
the Malpighian bodies. 

The next stage constitutes (Pl. VIII, fig. 9) the last which 


NOTES ON THE DEVELOPMENT OF THE ARANEINA. 99 


requires to be dealt with so far as the external features are 
concerned. The yolk has now mainly passed into the 
abdomen, and the constriction separating the thorax and 
abdomen has began to appear. The yolk sack has become ab- 
sorbed, so that the two halves of the ventral plate in the thorax 
are no longer widely divaricated. The limbs have to a large 
extent acquired their permanent structure, and the rings of 
which they are formed in the earlier stages are now replaced 
by definite joints. A delicate cuticle has become formed, 
which is not figured in my sections. The four rudimen- 
tary appendages have disappeared, unless, which seems to 
me in the highest degree improbable, two of them remain 
as the spinning mammille, which are now present. Be- 
hind is the anal lobe, which is much smaller and less con- 
spicuous than in the previous stage. The spinnerets and 
anal lobe are shown as five papille in Pl. VIII, fig. 9. Dor- 
sally the heart is now very conspicuous, and in front of the 
cheliceree may be seen the supra-cesophageal ganglia. 

The indifferent mesoblast has now to a great extent 
become converted into the permanent tissues. On the dorsal 
surface there was present in the last stage a great mass of 
unformed mesoblast cells. This mass of cells has now 
become divided into a somatic and splanchnic layer (Pl. X, 
fig. 22). It has, moreover, in the abdominal region at any 
rate, become divided up into somites. At the junction be- 
tween the successive somites the splanchnic mesoblast on 
each side of the abdomen dips down into the yolk and forms 
a septum (Pl. X, fig. 22s). The septa so formed, which 
were first described by Barrois, are not complete. The septa 
of the two sides do not, in the first place, quite meet along 
the median dorsal or ventral lines, and in the second place 
they only penetrate the yolk for a certain distance. In- 
ternally they usually end in a thickened border. 

Along the line of insertion of each of these septa there is 
developed a considerable space between the somatic and 
splanchnic layers of mesoblast. The parts of the body 
cavity so established are transversely directed channels pass- 
ing from the heart outwards. They probably constitute the 
venous spaces, and perhaps also contain the transverse aortic 
branches. 

In the intervals between these venous spaces the somatic 
and splanchnic layers of mesoblast are in contact with each 
other. 

I have not been able to work out satisfactorily the later 
stages of development of the septa, but I have found that 
they play an important part in the subsequent developmen. 


100 F. M. BALFOUR. 


of the abdomen. In the first place they send off lateral off- 
shoots, which unite the various septa together, and divide 
up the cavity of the abdomen into a number of partially 
separated compartments. There appears, however, to be 
left a free axial space for the alimentary tract, the meso- 
blastic walls of which are, I believe, formed from the septa. 

At the present stage the splanchnic mesoblast, apart from 
the septa, is a delicate membrane of flattened cells (fig. 22, 
sp). The somatic mesoblast is thicker, and is formed of 
scattered cells (so). 

The somatic layer is in part converted, in the posterior 
region of the abdomen, into a delicate layer of longitudinal 
muscles, the fibres of which are not continuous for the whole 
length of the body, but are interrupted at the lines of junc- 
tion of the successive segments. ‘They are not present in the 
anterior part of the abdomen. The longitudinal direction of 
these fibres, and their division with myotomes, is interesting, 
since both these characters, which are preserved in Scorpions, 
are lost in the abdomen of the adult Spider. 

The original mesoblastic somites have undergone quite as . 
important changes as the dorsal mesoblast. In the abdominal 
region the somatic layer constitutes two powerful bands of 
longitudinal muscles, inserted anteriorly at the root of the 
fourth ambulatory appendage, and posteriorly at the spinning 
mammille. Between these two bands are placed the nervous 
bands. The relation of these parts are shown in the section in 
Pl. X, fig. 20d, which cuts the abdomen horizontally and 
longitudinally. The mesoblastic bands are seen at m., and the 
nervous bands within them at ad. g. In the thoracic region 
the part of the somatic layer in each limb is converted into 
muscles, which are continued into dorsal and ventral muscles 
in the thorax (wde fig. 20c). There are, in addition to these, 
intrinsic transverse fibres on the ventral side of the thorax. 
Besides these muscles there are in the thorax, attached to 
the suctorial extremity of the stomodeum, three powerful 
muscles, which I believe to be derived from the somatic me- 
soblast. One of these passes vertically down from the dorsal 
surface, in the septum the commencement of which was 
described in the last stage. The two other muscles are 
lateral, one on each side (Pl. IX, fig. 20). 

The heart has now, in most respects, reached its full 
development. It is formed of an outer muscular layer, within 
which is a doubly-contoured lining, containing nuclei at 
intervals, which is probably of the nature of an epithelioid 
lining (Pl. X, fig. 22 ht). In its lumen are numerous 
blood-corpuscles (not represented in my figure). The heart 


NOTES ON THE DEVELOPMENT OF THE ARANEINA. 101 


lies in a space bound below by the splanchnic mesoblast, 
and to the sides by the somatic mesoblast. This space 
forms a kind of pericardium (fig. 22 pc), but dorsally the 
heart is in contact with the epiblast. The arterial trunks 
connected with it are fully established. 

The nervous system has undergone very important 
changes. 

In the abdominal region the ganglia of each side have 
fused together into a continuous cord (fig. 21 ad.g.). In 
fig. 20, in which the abdomen is cut horizontally and longi- 
tudinally, there are seen the two abdominal cords (ad. g.) 
united by two transverse commissures; and I believe that 
there are at this stage three or four transverse commissures 
at any rate, which remain as indications of the separate 
ganglia, from the coalescence of which the abdominal cords 
are formed. The two abdominal cords are parallel and in 
close contact. 

In the thoracic region changes of not less importance 
have taken place. The ganglia are still distinct. The two 
cords formed of these ganglia are no longer widely separated, 
but meet, in the usual way, in the ventral line. '‘Trans- 
verse commissures have become established (fig. 20 c, pd. g.), 
between the ganglia of the two sides. There is as little 
trace at this, as at the previous stages, of an imgrowth 
of epiblast, to form a median portion of the central nervous 
system. Such a median structure has been described by 
Hatschek for Lepidoptera, and he states that it gives rise 
to the transverse commissures between the ganglia. My ob- 
servations show that for the spider, at any rate, nothing of 
the kind is present. 

As shown in the longitudinal section (Pl. X, fig. 21), 
the ganglion of the chelicerze has now united with the supra- 
cesophageal ganglion. It forms, as is shown in fig. 20 6 (ch. 9.), 
a part of the cesophageal commissure, and there is no sub- 
cesophageal commissure uniting the ganglia of the chelicere, 
but the esophageal ring is completed below by the ganglia 
of the pedipalpi (fig. 20 c, pd. g.). 

The supra-cesophageal ganglia have become completely 
separated from the epiblast. 

I have unfortunately not studied their constitution in the 
adult, so that I cannot satisfactorily identify the parts which 
can be made out at this stage. 

I distinguish, however, the following regions : 

(1) A central region containing the commissural part, 
and continuous below with the ganglia of the chelicere. 

(2) A dorsal region formed of two hemispherical lobes. 


102 F, M. BALFOUR. 


(3) A ventral anterior region. 

The central region contains in its interior the commis- 
sural portion, forming a punctiform rounded mass in each 
ganglion : a transverse commissure connects the two masses 
(vide fig. 20 b). 

The dorsal hemispherical lobes are derived from the part 
which, at the earlier stage, contained the semicircular 
grooves. When the supra-cesophageal ganglia become sepa- 
rated from the epidermis the cells lining these grooves 
become constricted off with them, and form part of these 
ganglia. ‘Two cavities are thus formed in this part of the 
supra-cesophageal ganglia. These cavities become, for the 
most part, obliterated, but persist at the outer side of the 
hemispherical lobes (figs. 20 a and 21). 

The ventral lobe of the brain is a large mass shown in long 
section in fig. 21. It lies immediately in front of and almost 
in contact with the ganglia of the chelicere. 

The two hemispherical lobes agree in position with the 
fungiform body (pilzhutformige Korpern), which has attracted 
so much the attention of anatomists, in the supra-cesophageal 
ganglia of Insects and Crustacea; but till the adult brain of 
Spiders has been more fully studied it is not possible to state 
whether the hemispherical lobes become the fungiform bodies. 

Hatschek' has described a special epiblastic invagination 
in the supra-cesophageal ganglion of Bombyx, which is pro- 
bably identical with the semicircular groove of Spiders and 
Scorpions, but in the figure he gives the groove does not 
resemble that in the Arachnida. A similar groove is found 
in Peripatus, and there forms, as I have found, a large part 
of the supra-cesophageal ganglia. It is figured by Moseley, 
‘Phil. Trans.,’ vol. 164, pl. lxxv, fig. 9. 

The stomodeum is considerably larger than in the last 
stage, and is lined by a cuticle ; it is a blind tube, the blind 
end of which is the suctorial pouch of the adult. To this 
pouch are attached the vertical dorsal, and two lateral muscles 
spoken of above. 

The proctodeum (pr.) has also grown in length,and the two 
Malpighian vessels which grow out from its blind extremity 
(fig. 20 e, mp. g.) have become quite distinct. The part now 
formed is the rectum of the adult. The proctodeum is sur- 
rounded by a great mass of splanchnic mesoblast. The mesen- 
teron has as yet hardly commenced to be developed. There 
is, however, a short tube close to the proctodzum (fig. 20 e, 
mes), which would seem to be the commencement of it. It 


1 “ Beitrage z, Eutwick d. Lepidopteren,” ‘ Jenaische Zeit.,’ vol. xi 
ad g pidop ’ 
p. 124. 


NOTES ON THE DEVELOPMENT OF THE ARANEINA, 103 


ends blindly on the side adjoining the rectum, but is open 
anteriorly towards the yolk, and there can be very little doubt 
that it owes its origin to cells derived from the yolk. On 
its outer surface is a layer of mesoblast. 

From the condition of the mesenteron at this stage there 
can be but little doubt that it will be formed, not on the sur- 
face, but im the interior of the yolk. I failed to find any 
trace of an anterior part of the mesenteron adjoining the 
stomodeum. In the posterior part of the thorax (vzde fig. 
20 d), there is undoubtedly no trace of the alimentary tract. 

The presence of this rudiment shows that Barrois is 
mistaken in supposing that the alimentary canal is formed 
entirely from the stomodeum and proctodeum, which are 
stated by him to grow towards each other, and to meet at 
the junction of the thorax and abdomen. My own impres- 
sion is that the stomodzum and proctodzeum have reached 
their full extension at the present stage, and that both the 
stomach in the thorax and the intestine in the abdomen are 
products of the mesenteron. 

The yolk retains its earlier constitution, being divided 
into polygonal segments, formed of large yolk vesicles. 
The nuclei are more numerous than before. In the thorax 
the yolk is anteriorly divided into two lobes by the vertical 
septum, which contains the vertical muscle of the suctorial 
pouch. In the posterior part of the thorax it is undivided. 

I have not yet been able clearly to make out the eventual 
fate of the yolk. At a subsequent stage, when the cavity of 
the abdomen is cut up into a series of compartments by the 
growth of the septa, described above, the yolk fills these 
compartments, and there is undoubtedly a proliferation of 
yolk cells round the walls of these compartments. It would 
not be unreasonable to conclude from this that the compart- 
ments were destined to form the hepatic ceca, each cecum 
being enclosed in a layer of splanchnic mesoblast, and its 
hypoblastic wall being derived from the yolk cells. I think 
that this hypothesis is probably correct, but [ have met with 
some facts which made me think it possible that the thick- 
enings at the ends of the septa, visible in Pl. X, fig. 22, 
were the commencing hepatic ceca. 

I must, in fact, admit that I have hitherto failed to work 
out satisfactorily the history of the mesenteron and its appen- 
dages. The firm cuticle of young spiders is an obstacle both 
in the way of making sections and of staining, which I have 
not yet overcome, 


104 F. M. BALFOUR. 


General Conclusions. 


Without attempting to compare at length the develop- 
ment of the spiders with that of other Arthropoda, I propose 
to point out a few features in the development of spiders, 
which appear to show that the Arachnida are undoubtedly 
more closely related to the other Tracheata than to the 
Crustacea. 

The whole history of the formation of the mesoblast is 
very similar to thatin insects. ‘The mesoblast in both groups 
is formed by a thickening of the median line of the ventral 
plate (germinal streak). 

In insects there is usnally formed a median groove, the 
walls of which become converted into a plate of mesoblast. 
In spiders there is no such groove, but a median keel-like 
thickening of the ventral plate (Pl. IX, fig. 11), is very pro- 
bably an homologous structure. The unpaired plate of 
mesoblast formed in both insects and Arachnida is exactly 
similar, and beomes divided, in both groups, into two bands, 
one on each side of the middle line. Such differences as 
there are between Insects and Arachnida sink into insigni- 
ficance compared with the immense differences in the origin 
of the mesoblast between either group, and that in the 
Isopoda, or, still more, the Malacostraca and most Crustacea. 
In most Crustacea we find that the mesoblast is budded off 
from the walls of an invagination, which gives rise to the 
mesenteron. 

In both spiders and Myriopoda, and probably insects, the 
mesoblast is subsequently divided into somites, the lumen of 
which is continued into the limbs. In Crustacea mesoblastic 
somites have not usually been found, though they appear 
occasionally to occur, e.g. Mysis, but they are in no case 
similar to those in the Tracheata. 

In the formation of the alimentary tract, again, the 
differences between the Crustacea and Tracheata are equally 
marked, and the Arachnida agree with the Tracheata. 
There is generally in Crustacea an invagination, which gives 
rise to the mesenteron. In Tracheata this never occurs. 
The proctodeeum is usually formed in Crustacea before or, 
at any rate, not later than the stomodeum.! The reverse is 
true for the Tracheata. In Crustacea the proctodzeum and 
stomodeum, especially the former, are very long, and usually 
give rise to the greater part of the alimentary tract, while 
the mesenteron is usually short. 


1 If Grobben’s account of the development of Moina is correct this 
statement must be considered not to be universally true. 


NOTES ON THE DEVELOPMENT OF THE ARANEINA, 105 


In the Tracheata the mesenteron is always considerable, 
and the proctodzeum is always short. The derivation of the 
Malpighian bodies from the proctodeum is common to 
most Tracheata. Such organs are not found in the 
Crustacea. 

With reference to other points in my investigations, the 
evidence which I have got that the chelicere are true post- 
oral appendages supplied in the embryo from a distinct 
postoral ganglion, confirms the conclusions of most previous 
investigators, and shows that these appendages are equivalent 
to the mandibles, or possibly the first pair of maxilla of 
other Tracheata. The invagination, which I have found, of 
part of a groove of epiblast in the formation of the supra- 
cesophageal ganglia is of interest, owing to the wide exten- 
sion of a similar occurrence amongst the Tracheata. 

The wide divarication of the ventral nerve cords in the 
embryo renders it easy to prove that there is no median in- 
vagination of epiblast between them, and supports Kleinen- 
berg’s observations on Lumbricus as to the absence of this 
- invagination. I have further satisfied myself as to the 
absence of such an invagination in Peripatus. It is probable 
that Hatschek and other observers who have followed him 
are mistaken in affirming the existence of such an invagina- 
tion in either the Chetopoda or the Arthropoda. 

The observations recorded in this paper on the yolk cells 
and their derivations are, on the whole, in close harmony 
with the observations of Dohrn, Bobretsky, and Graber, on 
Insects. They show, however, that the first formed meso- 
blastic plate does not give rise to the whole of the mesoblast, 
but that during the whole of embryonic life the mesoblast 
continues to receive accessions of cells derived from the cells 
of the yolk. 


Araneina. 


1, Balbiani, “ Mémoire sur le Développement des Araneides,” ‘ Ann. 
Sci. Nat.,’ series v, vol. xvii, 1873. 

2. J. Barrois, “Recherches s. 1. Développement des Araignées,” 
‘Journal de 1. Anat. e. de la Physiol.,’ 1878. 
ae Li. Claparéde, ‘Recherches s, ? Evolution des Araignées,’ Utrecht, 

4. Herold, ‘De Génératione Araniorum in Ovo,’ Marburg, 1824, 

5. HA. Ludwig, “ Ueb. d. Bildung des Blastoderm,” bie d. Spinnen, 
‘Zeit. f. wiss, Zool.,’ vol. xxvi, 1876. 


106 F. M. BALFOUR. 


Postscript to the Paper on the Development of the 
Araneina 


Since the publication of the preceding paper, my attention 
has been called to a very important paper by Salensky, on the 
“ Development of the Araneina,” written in Russian, which ap- 
peared in 1871 in the ‘ Kiew Society of Naturalists.’ An abstract 
of this paper is published in Hofmann and Schwalbe’s ‘ Jahresber- 
ichte’ for 1878. From the abstract it appears that Salensky was 
able to carry his investigations.a good deal further than previous 
investigators. He detected the nuclei in the yolk, and showed that 
they gave rise in part to the inner layer of blastoderm cells. He 
further showed that the mesoblast became split into splanchnic 
and somatic layers, and gave an account of the development of 
the heart, similar to that in my paper. He believes that the 
third and fourth pairs of provisional abdominal appendages be- 
come the spinning mammille; while the anterior pair gives rise 
to the pulmonary sacs. I was myself unable to follow in detail 
the disappearance of the four provisional appendages, though 
from various considerations I was led to reject the view that 
the third and fourth pairs became the spinning mammille. I 
could not moreover trace the development of the pulmonary sacs 
from the first pair of abdominal appendages. Further ob- 
servation on these points with fresh material are much required ; 
and I hope myself, to be able before long, to return to this part 
of the development as well as to the history of the later stages. 


On the DEVELOPMENT of the STRUCTURE KNOWN as the 
‘ GLtomeRuLuS of the Huap-Kipney’ in the Caick. 
By Apam Sepewiox, B.A. 


In a paper by Mr. Balfour and myself (vide p. 1—20 
of “these Studies’’), describing the development of what we 
believed to be a rudimentary head-kidney in the chick, we 
drew attention to a structure which so closely resembled the 
glomerulus of the head-kidney of the Icthyopsida that we iden- 
tified it as an homologous structure. 

Gasser ' has also independently discovered and similarly iden- 
tified this structure. 

In the paper just referred to no attempt was made to trace 
the development of this glomerulus, but it was merely described 
as it appeared at its time of greatest development. 

The following description is taken from that paper : 

“In the chick the glomerulus is paired and consists of a vas- 
cular outgrowth or ridge projecting into the body cavity on each 
side at the root of the mesentery. It extends from the anterior 
end of the Wolffian body to the point where the foremost open- 
ing of the head-kidney commences. We have found it at a 
period slightly earlier than that of the first development of the 
head-kidney. In the interior of this body is seen a stroma with 
numerous vascular channels and blood corpuscles, and a vas- 
cular connection is apparently becoming established, if it is not 
so already, between the glomerulus and the aorta. The stalk 
connecting the glomerulus with the attachment of the mesentery 
varies in thickness in different sections, but we believe that the 
glomerulus is continued unbroken throughout the very consider- 
able region through which it extends. This point is, however, 
difficult to make sure of, owing to the facility with which the 
glomerulus breaks away. At the stage we are describing no 
true Malpighian bodies are present in the part of the Wolffian 
body on the same level with the anterior end of the glomerulus, 
but the Wolffian body merely consists of the Wolffian duct. At 
the level of the posterior part of the glomerulus this is no longer 


‘ *Sitzungsberichte der Gesellschaft zur Beférd d. gesam. Naturwiss.,’ 
No. 5, 1879. 


108 ADAM SEDGWICK, 


the case, but here a regular series of primary Malpighian bodies 
is present, and the glomerulus of the head-kidney may fre- 
quently be seen in the same section as a Malpighian body. In 
most sections the two bodies appear quite disconnected, but in 
those sections in which the glomerulus of the Malpighian body 
comes into view it is seen to be derived from the same formation 
as the glomerulus of the head-kidney.” 

The point which is left in doubt in the above description, viz. 
as to whether the glomerulus constitutes a continuous structure, 
is at once decided by a study of its development. 

I may here state that it is not a continuous structure, but 
consists of a series of external glomeruli, each of which corre- 
sponds and is continuous with the glomeruli of the Malpighian 
bodies found in this part of the trunk. 

I will commence the description of the development at the 
time when the segmental tubes have reached the stage of devel- 
opment figured by Kolliker! and myself.* At this stage each of 
them in the anterior region of the Wolffian body has the form of 
an S-shaped string, with a narrow opening into the body cavity, 
the lower limb of the S being formed by the intermediate cell 
mass, and the upper limb by a column of cells which connects 
the intermediate cell mass with the Wolffian duct. 

In the region where each external glomerulus is afterwards 
found the openings into the body cavity, which are homologous 
with the peritoneal openings of the segmental tubes in Elasmo- 
branchs, widen out very considerably, and a lumen is continued 
from them into the intermediate cell mass on the one hand, and 
on the other hand into the column of cells which forms the 
upper limb of the § and connects the intermediate cell mass with 
the Wolffian duct.® 

That part of the segmental tube which will afterwards become 
a Malpighian body is therefore, in the region where an external 
glomerulus will subsequently be formed, connected with the body 
cavity by a short tube. This tube rapidly widens out, especially 

2 ‘ Entwicklungsgeschichte des Menschen u. der hoheren Thiere,’ p. 201, 
2nd ed. 

3 2 ‘Development of the Kidney, &c.,’ p. 62 of- these Studies,” Pl. VI, 
g, 1. 
3 This may best be understood by examining fig. 11, Pl. VI, in my paper 
already referred to. Ifthe primary Wolffian tubule (zw?'), here represented, 
were connected with the peritoneal epithelium at the point where the line 
from w! cuts it, and it were open to the body-cavity at that point, an appear- 
ance similar to that which I have attempted to describe would be obtained. 
Or perhaps a better idea of the structure may be obtained from fig. 6, Pl. 
XX, in Balfour’s ‘ Monograph on the Development of Elasmobranch Fishes.’ 
If s¢ were very short and wide, so that mg were widely open to the body- 


cavity, the figure would resemble a developing Wolffian tubule in this 
anterior part of the chick’s Wolffian body. 


GLOMERULUS OF THE HEAD-KIDNEY. 109 


anteriorly, to such an extent that it soon appears as a shallow 
oay in the body cavity. Thus each opening at this stage forms 
a bay, wide and shallow anteriorly, becoming deeper and nar- 
rower as we pass backwards, until finally behind it is separated 
from the body cavity altogether, and there is seen in section a 
Malpighian capsule precisely resembling a developing Malpig- 
hian capsule in the hinder region of the Wolffian body.’ This 
bay and in the small chamber behind, continuous with the bay 
but separated from the body cavity, are together serially homo- 
logous with a Malpighian capsule and the funnel leading from 
it into the body cavity. In them asmall glomerulus soon appears 
attached to the dorsal wall. ‘The glomerulus increases in size, 
and the bay anteriorly widens out very much, while behind it 
remains deep, and finally passes into the closed posterior por- 
tion. The glomerulus fills up this passage which clearly runs 
obliquely backwards and dorsalwards, and eventually, as far as 1 
can ascertain, the opening becomes completely closed, the epi- 
thelium on the external glomerulus being no longer continued 
through the opening on to the internal glomerulus. 

The external glomerulus, then, in the chick which has hither- 
to been known as the glomerulus of the head-kidney, is nothing 
more than a series of glomeruli of primary Malpighian bodies 
projecting through the wide openings of the segmental tubes 
anto the body cavity. Their extreme antero-posterior extension 
may be said to be within the ninth and thirteenth segments. 

In the chick the primary segmental tubes corresponding to 
these external glomeruli are apparently never fully developed. 

I may mention that the external glomeruli are present in 
greater numbers and attain a greater development in the duck 
than in the chick. 

I defer the details and all discussion of this extraordinary and 
unexpected development until I am able to publish a fuller paper 
with figures. 


1 Loe. cit., fig. 11. 


DESCRIPTION OF PLATES I, II, 


Illustrating paper ‘‘On the Existence of a Head-Kidney 
in the Embryo Chick, and on Certain Points in the 
Development of the Miillerian Duct.” 


CoMPLETE List oF REFERENCE LETTERS. 


ao. Aorta. c.v. Cardinal vein. g/. Glomerulus. gr,. First groove of 
head-kidney. gr. Second groove of head-kidney. gr3. Third groove of 
head-kidney. g.e. Germinal epithelium. mr. 4. Malpighian body. me. 
Mesentery. m.d. Miillerian duct. 7,. First ridge of head-kidney. 7. 
Second ridge of head-kidney. 73. Third ridge of head-kidney. W. d. 
Wolffian duct. 2. Fold in germinal epithelium. 


EXPLANATION OF PLATE I. 


Serres A.—Sections through the head-kidney at our second stage. 
Zeiss, 2, ocul. 3 (reduced one third). The second and third grooves are 
represented with the ridge connecting them, and the rod of cells running 
backwards for a short distance. 

No. 1.—Section through the second groove. 

No. 2.—Section through the ridge connecting the second and third 
grooves. 

No. 3.—Section passing through the same ridge at a point nearer the 
third groove. 

Nos. 4, 5, 6.—Sections through the third groove. 

No. 7.—Section through the point where the third groove passes into 
the solid rod of cells. 

No. 8.—Section through the rod when quite separated from the germinal 
epithelium. 

No. 9.—Section very near the termination of the rod. 

No. 10.—Last section in which any trace of the rod is seen. 

Szries B.—Sections passing through the head-kidney at our third stage. 
Zeiss, C, ocul. 2. Our figures are representations of the following sections 
of the series, section 1 being the first which passes through the anterior 
groove of the head-kidney. 


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The Miillerian duct extends through eleven more sections. 

The first groove (gr,.) extends to No. 3. 

The second groove (gr,.) extends from No. 4 to No. 7. 

The third groove (gr3.) extends from No. 11 to No. 13. 

The first ridge (7,.) extends from No. 2 to No. 5. 

The second ridge (r,.) extends from No. 8 to No. 11. 

The third ridge (73.) extends from No. 13 backwards through twelve 
sections, when it terminates by a pointed extremity. 

Fic. C.—Section through the ridge connecting the second and third 
grooves of the head-kidney of an embryo slightly younger than that from 
which Series B. was taken. Zeiss, c, ocul. 3 (reduced one-third). 

The fold of the germinal epithelium, which gives rise to a deep groove 
(x.) external to the head-kidney is well marked. 

_ Serres G.—Sections through the rod of cells constituting the termina- 
tion of the Miillerian duct at a stage in which the head-kidney is still 
present. Zeiss, c, ocul. 2. 


EXPLANATION OF PLATE II. 


Series D.—Sections chosen at intervals from a complete series tra- 
versing the peritoneal opening of the Miillerian duct, the remnant of the 
head-kidney, and the termination of the Miillerian duct. Zeiss, c, ocul. 3 
(reduced one-third). 

Nos. ]. and 2.—Sections through the persistent anterior opening of the 
head-kidney (abdominal opening of Millerian duct). The approach of the 
Wolffian duct to the groove may be seen by a comparison of these two 
figures. In the sections in front of these (not figured) the two are much 
more widely separated than in No. 1. 

No. 3.—Section through the Millerian duct, just posterior to the per- 
sistent opening. 

Nos. 4 and 5.—Remains of the ridges, which at an earlier stage connected 
the first and second grooves, are seen passing from the Miillerian duct to 
the peritoneal epithelium. 

No. 6.—Rudiment of the second groove (g7,.) of the head-kidney. 

Between 6 and 7 is a considerable interval. 

No. 7.—All traces of this groove ( gr,.) have vanished, and the Miillerian 
duct is quite disconnected from the epithelium. 

No. 8.—Rudiment of the third groove (g73.). 

No. 9.—Millerian duct quite free in the space between the peritoneal 
epithelium and the Wolffian duct, in which condition it extends until near 
its termination. 

Between Nos, 9 and 10 is an interval of eight sections. 

No. 10.—The penultimate section, in which the Miullerian duct is seen. 
A lumen cannot be clearly made out. 

No. 11.—The last section in which any trace of the Miillerian duct is 
visible. No line of demarcation can be seen separating the solid end of the 
Miillerian duct from the ventral wall of the Wolffian duct. 

Fies. HE. and F.—Sections through the glomerulus of the head-kidney 
from an’embryo prior to the appearance of the head-kidney. Zeiss, B, 
ocul. 2. A comparison of the two figures shows the variation in the thick- 
ness of the stalk of the glomerulus. E.—Section anterior to the foremost 
Malpighian body. F.—Section through both the glomerulus of the head- 
kidney and that of a Malpighian body. ‘The two are seen to be connected. 

Serres H.—Consecutive sections through the hind end of the Miillerian 
duct, from an embryo in which the head-kidney was only represented by a 
rudiment. (The embryo was, perhaps, very slightly older than that from 
which Series D was taken.) Zeiss, c, ocul. 3 (reduced one third). 

No. 1.—Millerian duct is without a lumen, and quite distinct from the 
Wolffian wall. 

No. 2.—The solid end of the Miillerian duct is no longer distinct from 
the internal wall of the Wolffian duct. 

No. 3.—All trace of the Miillerian duct has vanished. 

Serres I.—Sections through the hinder erd of the Miillerian duct from 
an embryo of about the middle of the sixth day. Zeiss, c, ocul. 2 (reduced 
one third). 

No. 1.—The Miillerian duct is distinct and small. 

No. 2.—Is posterior by twelve sections to No.1. The Millerian duct 
is dilated, and its cells are vacuolated. 

No. 3.—Penultimate section, in which the Millerian duct is visible ; it 
is separated by three sections from No. 2. 

No. 4.—Last section in which any trace of the Millerian duct is visible ; 
the lumen, which was visible in the previous section, is now absent. 

No. 5.—No trace of Miillerian duct. Nos. 3, 4, and 5, are consecutive 
sections. 

Fie. K.—Section ,hrough the hind end of the abdominal opening of the 
Miillerian duct of a chick of 123 hours. Zeiss, c, ocul. 2 (reduced one- 
third). It illustrates the peculiar cord connecting the Miullerian and Wolffian 
ducts. ; 


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DESCRIPTION OF PLATE III, 


Illustrating paper on the “Early Development of the 
Lacertilia, together with some Observations on the 
Nature and Relation of the Primitive Streak.” 


Complete List of Reference Letters. 


m.g. Medullary groove. me. p. Mesoplastic plate. ep. Epiblast. 
hy. Hypoblast. ch’. Notochordal thickening of hypoblast. ch. Noto- 
chord. xe. Neurenteric canal (blastopore). pr. Primitive streak. am. 
Amnion. 


SERIES A.—Sections through an embryo shortly after the formation of the 
medullary groove. x 120.' 
~ Fic. 1.—Section through the trunk of the embryo. 
Fics. 2—5.—Sections through the neurenteric canal. 


Fic. 8.—Surface view of a somewhat older embryo than that from which 
Series ais taken. x 30. 


SERrEs B.—Sections through the embryo represented in Fig. B. X 120. 
Fic. 1.—Section through the trunk of the embryo. 
Fics. 2, 3.—Sections through the hind end of the medullary groove. 
Fie. 4.—Section through the neurenteric canal. 
Fic. 5.—Section through the primitive streak. 


Fie. c.—Surface view of a somewhat older embryo than that represented 
in Fie. zB. x 30. 


Deen ———— 


The spaces between the layers in these sections are due to the action 
of the hardening reagent. 


EXPLANATION OF PLATES IV AND V, 


Illustrating the Memoir on some Points in the Early 
Development of the Commor Newt (TZvriton teniatius), 
by W. B. Scott, B.A., and Henry F. Osborn, B.A. 


With the exception of fig. 1 the following figures were drawn with a 
Zeiss’ A objective. In figs. 2, 3, 4, 5, a No. 2 (Zeiss) eyepiece was used, 
and for figs. 6 and 7 a No. 3 eyepiece. 


EXPLANATION OF PLATE IV. 
List oF REFERENCES. 


ep. Epiblast. ep . Inner layer of epiblast. yé. Yolk. hy. Hypo- 
blast. in. hy. Invagination hypoblast. y. Ay. Yolk hypoblast. Mm. 
Mesoblast. sp. Splanchnopleure. so. Somatopleure. al. Alimen- 
tary canal. ac. Neural canal. ch. Notochord. mg. Medullary 
groove. mf. Medullary folds. 


Fic. 1.—Longitudinal section of an embryo at time of commencement of 
invagination. Hartnack No.7 obj., eyepiece 3. It shows one of the 
earliest stages of the epiblast. 

Fic. 2.—Represents a longitudinal section of a Triton embryo (probably 
cristatus) in the early part of Stage a. At the opening of the blastopore 
the section is in the median line. It slants off forwards, however, to one 
side, and therefore out of the region of the alimentary canal. It shows 
the formation of the invagination-hypoblast and the confused mass of cells 
arising from the reflection of the epiblast. 

Fic. 3.—A section of the same embryo. It may be considered the re- 
verse of the last. At the blastopore it is at one side of the median line, 
while anteriorly it is directly in the median line. This obliquity explains 
the apparent upgrowth of yolk-cells in the centre. Putting this and the 
previous section together, a fair idea may be obtained of the actual relation 
of the layers at this period. It illustrates the formation of mesoblast 
by invagination, and the obliteration of the segmentation cavity by 
the advance of the alimentary canal. The blastopore has been artificially 
widened. 

Fic. 4.—An anterior transverse section of an embryo, at Stage 4, slightly 
more advanced than the previous one. It shows the shallow medullary 
groove, the lateral plates of mesoblast extending half way down the sides, 
also the invagination-hypoblast above the alimentary canal continuous at the 
sides with the yolk hypoblast. 

Fic. 5.—A transverse section through the head region of an embryo of 


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EXPLANATION OF PLATE IV—Continued. 


Stage B. It shows the splitting of the mesoblast and the formation of the 
medullary plate and notochord. 
Fic. 6.—A_ transverse section through the trunk region of an embryo at 
Stage c, showing a slightly more advanced development than the last. 
Fic. 7.—Represents a transverse section through the anterior trunk 
_ region late in Stage D. 


EXPLANATION OF PLATE V. 
List oF REFERENCES. 


op. Optic vesicle. pp. Head cavities (numbered in order 1, 2, &c.) 
ve. Visceral clefts. aa. Aortic arches and auditory vesicles. eb, Ex- 
ternal branchia. mb. Mid brain. hb. Hind brain. th. Thyroid 
body. al. Alimentary canal. ep. Outer layer of epiblast. ep’. Inner 
layer of ditto. Zeiss. A, obj. oc. No. 2, exeept for figs. 9, 16, and 17. 


Fie. 8.—Another transverse section in the middle region. This section 
is cut obliquely, so that the lateral and vertebral plates of mesoblast do not 
appear continuous with the mesoblast lining the sides of the embryo; it gives 
therefore at first sight a false impression. 

Fic. 9.—Enlarged view of the lateral epiblast of fig. 6. Zeiss D, ocul. 
3. a. One point of cell division. 

Fie. 10.—Horizontal longitudinal section through the head of an embryo 
of Stage Fr. The section is slightly oblique, and hence unsymmetrical. It 
shows the unsegmented head cavity. 

Fie. 11.—Vertical longitudinal section through the head of an embryo of 
Stage K, showing the relations of the head cavities, aortic arches, and gill 
clefts ; it is taken too much at the side to show the thyroid. 

Fic. 12.—Transverse section through head of an embryo of Stage 1. 

Fic. 13.—Transverse section of head of embryo very slightly older than 
the preceding figure. 

Fic. 14.—Section through the same embryo as fig. 12, but considerably 
further forwards. 

Fic. 15.—Transverse section through the head of an embryo of about 
Stage m. 

Fie. 16.—External drawing of an embryo of Stage p. s. 7. Sinus 
rhomboidalis. 

Fie. 17.—External drawing of an embryo of Stage 1. o. Oral invo- 
lution. 


EXPLANATION OF PLATES VI AND wil, 


Illustrating Mr. Sedgwick’s Memoir on “ Development of 


the Kidney in its Relation to the Wolffian Body in the 
Chick.” 


Complete List of Reference Letters. 


Ao. Aorta. Al. Alimentary canal. cl. Cloaca. cc. v. Cardinal vein. 
ep. Kpiblast. hy. Hypoblast. 7. c. m. Intermediate cell mass. 17. ¢. m.! 
Cell mass, which later becomes the intermediate cell mass. m.e. 
Mesentery. U.d. Miillerian duct. 4&. 6. Kidney blastema. 4. ¢. 
Kidney tubule. x. c. Notochord, jp. Protovertebra. p’. Cell mass, 
which later becomes a protovertebra. p.v. Body-cavity. p.e. Peri- 
toneal epithelium. 7. Testis. «. Ureter. ve. Vertebral body. w. 
Wolffian body. w. 6. Wolffian blastema. w.d. Wolffian duct. vw. ¢.' 
Primary Wolffian tubule. w. ¢.? Secondary ditto. w. 7.3 Tertiary ditto. 


Fig. 1.—Section between the fifteenth and sixteenth protovertebrze 
of a chick with twenty-three protovertebre, showing the rudimentary 
continuation of the body-cavity into the intermediate cell mass and the 
connection which the latter has obtained with the Wolffian duct. The 
intermediate cell mass in anterior and posterior neighbouring sections 
has separated from the peritoneal epithelium, 


Fies. 2, 3, 4, and 5.—Sections taken from a duck embryo with about 
thirty-two protovertebre, illustrating the development of the Wolflian 
tubules. Hart. cam., ob. 4. 


Fie, 2.—Section through the thirtieth segment, intermediate cell mass 
continuous with peritoneal epithelium, and containing a rudimentary 
prolongation of the body-cavity. Lumen of Wolffian duct doubtful. 


Fic. 3.—Section through the twenty-ninth segment, intermediate 
cell mass separate from peritoneal epithelium. 


Fic. 4.—Section through the twenty-sixth protovertebra, showing 
features similar to above. 


Fic. 5.—Section through the twenty-second protovertebra ; com- 
mencing differentiation of Wolffian tubule. 


Fies. 6—10.—Sections illustrating the more modified development of 
the Wolffian blastema, as seen in the chick behind the twentieth segment. 


Fig. 6.—Section through a chick with twenty-six protovertebre 
behind the last-formed segment, showing the thick peritoneal epi- 
thelium, the Wolffian blastema in connection with the mass of cells 
which will become a protovertebra. Hart. cam., ob. 4. 


Fic. 7.—Section through the twenty-ninth protovertebra of a chick 
with twenty-nine protovertebre, showing the thick peritoneal epi- 
thelium and the Wolffian blastema in connection with the protovertebre. 
Hart. cam., ob. 4. 

Fie. 8.—Section through the twenty-fourth segment of a chick with 
twenty-six protovertebrz, showing the Wolffian blastema separate from 
protovertebre and thick peritoneal epithelium. Hart. cam., ob. 3. 


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EXPLANATION OF PLATES VI AND VII—Continued. 


Fie. 9—Section through the twenty-fourth segment of a chick with 
twenty-nine protovertebrz, showing Wolffian blastema and thin peri- 
toneal epithelium. Hart. cam., ob. 3. 

Fie. 10.—NSection through the twenty-ninth segment of a chick with 
thirty-four protovertebrz, showing the commencing development of a 
primary Wolflian tubule from Wolffian blastema. Hart. cam., ob. 4. 

Fig. 11.—Section through a chick, end of third day or beginning of 
fourth, showing earliest appearance of a secondary tubule. Hart, 
cam., ob. 3. 

Fie, 12.—Section through the thirty-second protovertebra of a 
chick with thirty-four protovertebre, showing the kidney blastema. 

Fies. 13—17.—A series of sections from the hind end of a chick of 
fourth day, illustrating the continuity of the Wolffian body with the 
cells forming the kidney blastema. Hart. cam., ob. 3. 

Fig. 13.—Last section, in which a tertiary tubule was seen. 


Fie. 14.—Last section, in which a secondary tubule was seen. The 
tubules in figs. 13 and 14 are contiguous. 


Fie, 15.—-Next section but one behind fig, 14. 
Fic. 16.—Next section but one to fig. 15. 
Fic. 17 a.—Section some distance behind that drawn in fig. 16. 


Fies. 15, 16, and 17 show kidney blastema. 
Fig. 17 shows opening of Wolffian duct into horn of cloaca. 


Fries. 18—20.—Sections through a slightly older embryo than that 
from which above series was taken. Hart. cam., ob. 3. Showing (fig. 
20), ureter opening into Wolffian duct, with shifted kidney blastema 
lying just internal to it. 

Fie. 19.—Showing developing ureter and kidney blastema. 

Fic. 20.—Section just anterior to ureter through the anterior end of 
the kidney blastema, 


Fic, 21.—A longitudinal vertical section through the hind end of a 
four-day chick, showing continuity of kidney blastema with hindermost 
part of Wolffian blastema, in which the development of Wolffian tubule 
is taking place. No line of demarcation can be drawn between the two. 


Fic. 22.—Section through a chick of seventh day or late in sixth, 
showing the portion of the ureter (w.) and iis dorsal dilatation (v. 4.) 
with regard to the Wolffian body (w.). 

ee 23 and 24 are from sections of the chick from which fig. 22 was 
taken. 

Fie, 23.—Section next but one to fig. 22. It shows the kidney tubule 
dorsal to the ureter, surrounded by the blastema. 

Fig. 24.—Section next to fig. 23. It shows the dilated termination of 
the kidney tubule, and the continuity of its lining cells with those of the 
kidney blastema, 

Fic. 25.—From a section through the kidney of an eight-day chick, 


showing the termination of a kidney tubule. It presents the same 
feature as fig. 24, 


EXPLANATION OF PLATES VIII, IX, AND X, 


Illustrating Mr. F. M. Balfour’s Notes on the Develop- 
ment of the Araneina, 


PLATE VIII. 
Complete List of Reference Letters. 


ch. g. Ganglion of chelicere. c. 7. Caudal lobe. ch. Chelicere. pd. 
Pedipalpi. pr. /. Preoral lobe. pp". pp*. etc. Provisional appendages. 
p.¢c. Primitive cumulus. sp. Spinnerets. s¢. Stomodzum. 


I—IV. Ambulatory appendages. 1—6. Postoral segments. 

Fie. 1.—Ovum, with primitive cumulus and streak proceeding from it- 

Fie. 2.—Somewhat later stage, in which the primitive cumulus is still 
visible. Some distance from it is a white area, which is probably the 
rudiment of the procephalic lobe. 

Fic. 3a and 36.—View of an embryo from the ventral surface and from 
the side when six segments have become established. 

Fie. 4.—View of an embryo, ideally unrolled, when the first rudi- 
ments of the appendages become visible. 

Fic. 5.—Embryo ideally unrolled at the stage when all the appendages 
have beeome established. 

Fie. 6.—Somewhat older stage, when the limbs begin to be jointed. 
Viewed from the side. 

Fic. 7.—Later stage, viewed from the side. 

Fic. 7a.—Same embryo as fig. 7, ideally unrolled. 

Fic. 8a and 84.—View of an embryo from the ventral surface and 
from the side, after the ventral flexure has considerably advanced. 

Fic. 9.—Somewhat older embryo, viewed from the ventral surface. 


PLATES IX AND X. 
Complete List of Reference Letters. 


ao. Aorta. ab. g. Abdominal nerve cord. ch. Chelicere. ch. g. Gan- 
glion of chelicerz. ep. Epiblast. At. Heart. 4s. Hemispherical lobe 
of supra-esophageal ganglion. /. 7. Lowerlip. m. Muscles. me. Meso- 
blast. mes. Mesenteron. mp.g. Malpighian tube. ms. Mesoblastic 
somite. @. Gsophagus. p.c. Pericardium. pr. Proctodeum (rectum), 
pd. Pedipalpi. pd. g. Ganglion of pedipalpi. pr. c. Primitive cumulus. 
s. Septum in abdomen. so. Somatopleure. sp. Splanchnopleure.  s¢. 
Stomodeum, sw. Suctorial apparatus. sw. g. Supra-esophageal ganglion. 
th. g. Thoracic ganglion. v.g. Ventral nerve cord. ys. Yolk. y.c. 
Cells derived from yolk. y.z. Nuclei of yolk cells. 


I g—IlV g. Ganglia of ambulatory limbs. 1—16. Postoral segments. 


PLATE IX & X.—Continued. 


Fic. 10.—Section through an ovum, slightly younger than fig. 1. 
Showing the primitive cumulus and the columnar character of the cells 
of one half of the blastoderm. 


Fie. 11.—Section through an embryo of the same age as fig. 2. 
Showing the median thickening of the blastoderm. 


Fie. 12.—Transverse section through the ventral plate of a somewhat 
older embryo, Showing the division of the ventral plate into epiblast and 
mesoblast. 


Fic. 13.—Section through the ventral plate of an embryo of the same 
age as fig. 3, showing the division of the mesoblast of the ventral plate 
into two mesoblastic bands. 


Fic. 14.—Transverse section through an embryo of the same age as 
fiz. 5, passing through an abdominal segment above and a thoracic 
segment below. 


Fic. 15—Longitudinal section slightly to one side of the middle line 
through an embryo of the same age. 


Fic. 16.—Tranverse section through the ventral plate in the thoracic 
region of an embryo of the same age as fig. 7. 


Fig. 17.—Transverse section through the procephalic lobes of an 
embryo of the same age. gr. Section of hemicircular groove in pro- 
cephalic lobe. 


Fig. 18,—Transverse section through the thoracic region of an 
embryo of the same age as fig. 8. 


Fie. 19.—Section through the procephalic lobes of an embryo of the 
same age. 


Fie. 20 a, b,c, d, e-—Five sections through an embryo of the same 
age as fig. 9. «@ and @ are sections through the procephalic lobes, 
e through the front part of the thorax. d cuts transversely the 
posterior parts of the thorax, and longitudinally and horizontally the 
ventral surface of the abdomen. e cuts the posterior part of the ab- 
domen longitudinally and horizontally, and shows the commerfcement 
of the mesenteron. 


Fig. 21.—Longitudinal and vertical section of an embryo of the same 
age. The section passes somewhat to one side of the middle line, and 
shows the structure of the nervous system. 


Fig. 22.—Transverse section through the dorsal part of the abdomen 
of an embryo of the same stage as fig. 9. 


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STUDIES 


MORPHOLOGICAL LABORATORY 


IN THE 


UNIVERSITY OF CAMBRIDGE, 


EDITED BY 


F. M. BALFOUR, M.A., F-.R.S., 


FELLOW OF TRINITY COLLEGE, CAMBRIDGE. 
PART IT. 


WILLIAMS AND NORGATE, 
14, HENRIETTA STREET, COVENT GARDEN, LONDON 
AnD 20, SOUTH FREDERICK STREET, EDINBURGH. 


1882. 


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LONDON: 
PRINTED BY J. E. ADLARD, BARTHOLOMEW CLOSE. 


CONTENTS. 


PAGE 
*Mr. Sypnry J. Hicxson.—The Eye of Pecten. Plates 
Land 1 : ; I 


*Mr. Apam Szpewick.—On the Harly Development of, 
the Anterior Part of the Wolffian Duct and Body. 
in the Chick, together with some Remarks on the 
Excretory System of the Vertebrata. Plate III 13 


*** MR, F. M. Batrour.—On the Development of the 
Skeleton of the Paired Fins of Elasmobranchii, 
considered in relation to its bearings on the 
Nature of the Limbs of the Vertebrata. Plates 
IVandV. 51 


*Mr, F. M. Batrour.—On the N ature of the aang in 
Adult Teleosteans and Ganoids, which is usually 


regarded as the Head Kidney or Pronephros . 69 

*Mr. K. Mitsvuxuri.—On the Development of the 
Suprarenal Bodies in Mammalia. Plate VI :. fe 

**Mr. F. M. Batrour and W. N. Parxrer.—On the 
Structure and Development of Lepidosteus . = 

**Mr. Apam SEep@wick.—On Certain Points in the 
Anatomy of Chiton : é ; . we 

**Mr. Watter Heare.—On the Germinal Layers and 
Early Development of the Mole . . 107 


*Mr. F. M. Batrour and Mr. F. Dutenron.—A 
Renewed Study of the Germinal Layers of the 
Chick. Plates VII, VIII, and IX : 117 


N.B.—The papers marked with one asterisk are reprinted from 
the ‘Quarterly Journal of Microscopical Science,’ the papers 
with two asterisks are reprinted from the ‘ Proceedings of the 
Royal Society,’ and the paper with three asterisks from the 
‘ Proceedings of the Zoological Society.’ 


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EXPLANATION OF PLATES I & Il, 


Illustrating Mr. Sydney J. Hickson’s Memoir on the 
‘Hye of Pecten.” 


Fic. 1.—A diagrammatic sketch of an eye of Pecten maximus. a. The 
cornea. 0. The transparent basement membrane supporting the epithelial 
cells of the cornea. c. The pigmented epithelium. d. The lining epi- 
thelium of the mantle. e. Thelens. . The ligament supporting the lens. 
g. The retina. #. The tapetum. &. The pigment. /. The optic nerve. 
m. The retinal nerve. m. Complementary nerve. y. The circumpaleal 
nerve (Duvernoy). g. A supplemental nerve from pedal ganglion. 


Fig. 2.—Epithelial cells of cornea. 


Fig. 3.—Section through the junction of the pigmented epithelium with 
corneal epithelium. 


Fic. 4.—Vertical section through eye of Pecten maximus. a, b. Cornea. 
c. Pigmented epithelium. d. Mantle epithelium. e. Lens. g. Retina. 
h. Tapetum. &. Pigment. 7. Section of optic nerve. 


Fics. 5, 6.—Isolated rods. 6a.—Diagrammatic- sketch of a central 
rod. ./. Posterior limb. m.p. Membrane pierced by rods. a./. Anterior 
limb. s.7. Spindle-shaped portion of rod. z. Nerves. 


Fies. 7.—Transverse sections through eye of P. maximus. a. Rods in 
section. 4. Tapetum. c¢. Pigment. d. Retinal nerve. 


Fic. 8.—Vertical section of the eye of Pecten maximus, showing the 
nerve dividing into retinal and complementary branches. 


Fie. 9.—Vertical section of the eye of Pecten maximus, showing the 
termination of the retinal nerve. The retina has dropped out, and the 
frayed-out end of the nerve remains. 

Fre. 10.—Section of eye of Pecten opercularis. 

Fre. 11. Retina (a) of P. opercularis, (6) of P. jacobeus, (c) P. maximus. 

Note.—A horizontal section means a section made in the same plane as 
the mantle. A transverse section, in a plane at right angles to the eye 


stalk. A vertical section, in a plane at right angles, both to the plane of the 
mantle and the last-named plane. 


3 


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The Eye of Pecren. By Sypney J. Hickson, B.Sc., 
Scholar of Downing College, Cambridge. (With Plates 
I and II.) 


THe general absence of organs of vision amongst the 
members of the class Lamellibranchiata meets with a curious 
and interesting exception in the genera Pectenand Spondylus. 

These genera have long been known to possess a great 
number of eyes of considerable complexity, situated on the 
border of the mantle. The number of these eyes varies 
considerably in different individuals, ranging in the genus 
Pecten from eighty to one hundred and twenty. ‘Their 
position also varies ; for, although they are always situated 
on the border of the mantle, yet sometimes they are placed 
at equal distances from one another, and sometimes they are 
clustered together in certain localities. 

Notwithstanding this indefinite element, both in their 
number and position, which might be expected to run paral- 
lel with a primitive and simple organisation, their anatomy is 
exceedingly complicated, and exhibits all the most important 
structural elements of the eyes of the higher Vertebrata. 

The earliest investigations into the anatomy of Pecten’s 
eye are those of Krohn,! who gives a drawing of the course 
of the optic nerve. This drawing is copied in many of the 
subsequent papers on the subject by other investigators, and, 
as far as it goes, is correct. Duvernoy,” in his description 
of the nervous system of the Pectens, gives a short descrip- 
tion of the anatomy of the eye. This paper, however, is 
chiefly valuable for the excellent figures and descriptions 
of the distribution of the nerves in the mantle, and the 
filaments which are given off from the main trunks of these 
to supply the tentacles and the eyes. 

‘The researches of Blanchard® and of Kefersteint which 
followed did not add very much to our knowledge on this 
subject, and it was not until 1865 that any careful histo- 
logical inquiries were carried on. It was Hensen’ who first 


' Krohn, ‘ Miiller’s Archiv,’ 1840, p. 301, pl. xi. 

2 Duvernoy, ‘Mémoires de l’Academie de Sciences,’ t. xxiv, 1852. 
‘Mémoire sur le systéme nerveux des Acephalés,’ p. 73, pl. ii. 

3 Blanchard, ‘ Organisation du regne animal: Mollusques Acephalés,’ 

4 Kerferstein, ‘ Zeit. fiir wiss. Zoologie, 1863, p. 133. 

5 Hensen, ‘ Zeit. fiir wiss. Zoologie,’ 1865, p. 220. 


2 SYDNEY J. HICKSON. 


gave figures of the characters of any of the histological 
elements. But as the eye of Pecten forms only a very small 
part of the paper, his figures and description are by no 
means complete, and in many respects they are incorrect. 
Finally, J. Chatin! has contributed two short papers, with- 
out figures, on this subject. 

Of the scanty literature Hensen’s paper is by far the 
most important, and he alone gives any good figures of 
sections of the eye, or of its elements; the other observers 
give remarkably few figures, and consequently I have had, 
owing to an imperfect knowledge of the German language, 
some difficulty in making myself acquainted with the sub- 
stance of their papers. 

I have been encouraged in publishing the following re- 
searches chiefly by this scarcity of good figures, but also 
because I believe, and will give my reasons for believing, 
that these eyes deserve more mention than is usually made 
of them in our zoological text-books. 

My investigations were chiefly carried on upon Pecten 
maximus, but I have also had the opportunity of making 
sections of and studying the eyes of two other species, 
Pecten gacobeus and Pecten opercularis. The eyes of these 
three species differ from one another in one or two not 
altogether unimportant particulars, and, as I shall after- 
wards point out, they form an interesting gradation, the 
points of difference between P. mazimus and P. oper- 
cularis passing through intermediate stages in P. jacobeus. 

The eyes of Pecten maximus—are situated amidst a number 
of tentacles, which run all round the border of the mantle. 
These tentacles are capable of considerable movement, and 
frequently overhang the eyes ard protect them from the 
light. ‘The eyes themselves are situated upon short stalks, 
which resemble very closely the basal part of an ordinary 
tentacle. 

This similarity caused Duvernoy to name a tentacle a 
tactile pedicel, and an eye an ocular pedicel, thus to a cer- 
tain extent implying that they are morphologically homo- 
logous organs respectively modified for a tactile and an 
ocular function. This homology is justified by certain 
points in their anatomy, such as the course of the nerve and 
the arrangement of the muscular fibres, and I believe that 
when the development of these eyes is studied the homology 
will be still further confirmed. 

The border of the mantle which bears the tentacles and 


1 J. Chatin, ‘ Bulletin de la Société Philomatique.’ Paris, 1877. 


THE EYE OF PECTEN. 3 


eyes is covered with an epithelium, consisting of columnar, 
non-ciliated, and slightly granular cells bearing nuclei, 
situated near the base of the cells. As this epithelium 
passes over the eye-bulbs, it undergoes two interesting modi- 
fications. It becomes considerably thicker and filled with a 
dark brown pigment (Pl. I, fig. 1 ¢) as it passes round 
the sides of the eyes, but immediately in front of the 
eye (Pl. I, fig. 1 @), it again diminishes in thickness, 
and becomes perfectly transparent. By thus surrounding 
the eye on all sides with a dark-coloured pigment, leaving 
only a round spot in front, clear and transparent, the epi- 
thelium, by limiting the entrance of the light to a small 
diaphragm in front, here performs the function of an iris. 
The epithelium which runs over this transparent part, and 
which forms the epithelial layer of the cornea, differs from 
the ordinary epithelium covering the rest of the mantle in 
that their cells are rather larger, are perfectly transparent in 
the living condition, and the nuclei are large and spherical, 
and situated in the centre of the cells. 

The eye consists of the following parts, which I shall now 
describe in order. The cornea, covered externally by its 
transparent epithelium, protects a large elliptical lens. 
Close up to the lens is the retina, but separated from it by 
the optic nerve, which spreads out over the anterior surface 
of the retina. The retina rests upon a tapetum, and behind 
this, occupying all the posterior concavity of the eye-cup, 
there is a red pigment. 

The cornea—consists of two parts, the outer epithelium, 
which has already been described, and a basement mem- 
brane, consisting of a thin layer of connective tissue. As 
before stated, this epithelium is merely a modification of the 
general epithelium of this part of the mantle; and the pig- 
mented epithelium surrounding the eye-bulbs (in like manner, 
a modification of the same tissue) is continuous with it 
all round its edge. The passage of the cells of the pig- 
mented epithelium into those of the corneal epithelium is 
signalised by two important changes in the characters of the 
cells. In the first place the pigment entirely disappears, 
and the nuclei, which in the former case were obscured by 
the pigment, now become apparent, and in the second place 
the cells are considerably diminished in their longitudinal 
axis. ‘The diminution in size of the cells causes the edge of 
the cornea to be sunk below the level of the pigmented epi- 
thelium ; and a shallow trough runs round the line of its 
juncture with it (Pl. I, fig. 3). The convexity of the 
cornea is not great, and the dome of it frequently only just 


4 SYDNEY J. HICKSON. 


reaches the level of a line drawn from the highest points of 
the pigmented epithelium on either side of it. ‘This appear- 
ance is not often seen in sections, as the pigmented epi- 
thelium rapidly shrinks, when the tissue dies, and under 
most reagents ; but I am fully persuaded of the accuracy of 
this statement from an examination of the eyes of living 
specimens of Pecten maximus and sections of Pecten oper- 
cularis. 

- The delicate epithelial cells of the cornea, in consequence 
of being entirely unprotected by any membrane similar to 
the conjunctiva of the higher animals, are quite naked, and 
very liable to injury from the rough edges of the tentacles 
which surround them. The arrangement just described, 
however, probably prevents the tentacles from coming into 
immediate contact with them. The little trough which runs 
round the margin of the cornea always contains a little 
liquid, even when the eye itself is removed from the water ; 
and the pressure of the tentacles when folding over the eye 
causes it to spread out as a thin layer over the cornea, and 
thus the cells are prevented from coming into immediate 
contact with the tentacle. 

Thus, the two remarkable modifications, namely, the pre- 
sence of a large quantity of pigment, and a greater longi- 
tudinal axis of the cells which the pigmented epithelium 
exhibits, are of considerable value to the eye, firstly, to pre- 
vent very divergent rays from entering, and secondly, to pre- 
vent any damage to the cornea caused by the rubbing of the 
adjacent tentacles over the sides of the eye. 

The second layer of the cornea is about half as thick as 
the epithelial layer, and, like it, is perfectly colourless and 
transparent. It consists merely of a thin continuation of 
the connective tissue of the stalk. It may be called the 
basement membrane of the corneal epithelium, as from the 
absence of any definite cellular elements its only function 
probably is to support these cells. 

Beyond the cornea this membrane becomes much thicker, 
and supports the pigmented epithelium, and at the same 
time structural elements make their appearance in it. From 
thence it passes into the connective tissue of the eye-stalk 
without further modifications. 

The /ens—is one of the most interesting parts of the eye. 
It is comparatively large, and is composed of a number of 
nucleated cells. In the fact that the lens is formed by 
more than one cell the eye of Pecten bears an interesting 
resemblance to that of the Vertebrata. The shape of the lens 
has been a subject of much dispute amongst the authors 


THE EYE OF PECTEN, 5 


who have written on this subject. Krohn and Keferstein 
believed it to be spherical. Hensen has figured it filling up 
the space between the cornea and retina, and consequently 
of an irregular bi-convex shape. 

It is difficult to see how a controversy on such a simple 
subject could have arisen, unless it is because different 
authors have examined different species, and described them 
for the genus. 

As regards Pecten maximus,an examination of the fresh eye 
has convinced me that in this species the lens is elliptical, the 
major axis being parallel to the plane of the mantle. A section 
of the eye made in a plane at right angles to the plane of the 
mantle and the direction of its margin—that is, the plane 
which is most convenient for section-cutting, and the one 
which is apparently usually adopted—would consequently 
cause the lens to appear circular in section. In the dia- 
grammatic representation of the eye (fig. 1) I have for con- 
venience sake represented the lens as being at right angles 
to the plane of the mantle in order that the true shape of 
the lens may not be overlooked. 

A fresh examination of the lens, when teased out from the 
rest of the eye, exhibits one or two interesting points. The 
lens is not, as in most eyes, perfectly colourless, but possesses 
a well-marked brown colouration, and a number of fine striz 
may be seen running in the direction of the major axis. 
The lens does not appear so perfectly elliptical in the fresh 
condition as in certain sections I have made; it is drawn 
out somewhat longitudinally, so as to be more like a double 
cone than an ellipse. This is probably due to the leus being 
released from the ligaments and connective-tissue pressures, 
which cause it to retain its proper shape. 

Hensen says that the lens is very soft, and the cells are 
light, polygonal, and nucleated. A careful examination 
of the lens of P. maximus has led me to avery different con- 
clusion. The lens seemed to be of exactly the same nature 
as in the higher forms, and when teasing it out I found some 
difficulty in holding it with a needle, as it slipped away from 
under it when a slight pressure was exerted. As regards 
the shape of the cells composing the lens, they are not all 
polygonal, as would be inferred from Hensen’s remarks on 
the subject. In the centre they are polygonal, but as they 
approach the periphery they become more and more flat- 
tened and elongated, until at the periphery they are strap- 
shaped. They are nucleated. As Hensen, I could find no 
membrane covering the lens, and no muscular fibres con- 
nected with it; but in a few cases I have observed a liga- 


6 SYDNEY J. HICKSON. 


ment, such as I have represented diagrammatically in 
fig. 1 f, which, I believe, forms a support for the lens. This 
ligament is usually broken by the action of reagents, and 
then hangs down by the side of the cavity, and thus becomes 
difficult to observe; at the same time the lens sinks down, 
and rests upon the anterior surface of the retina. 

The lens is suspended in the space which corresponds 
with the vitreous humour in the higher animals. This space 
is filled with an aqueous humour in Pecten. The lens is 
larger, and, consequently, the space occupied by aqueous 
humour relatively smaller in P. maximus than itis in either 
P. jacobeus or P. opercularis, and in P.jacobeus it is larger 
than in P. opercularis. 

The retina—does not line the concavity of the eye-cup, as 
it does in most well-developed eyes, but is nearly flat, and 
a considerable space is left between it and the floor of the 
cup, which is filled up by the red pigment. In conse- 
quence of this the retina appears in section to be a thick 
band crossing the eye from side to side. Thus, just as the 
lens was remarkable for the way in which it approached the 
retina by hanging back into the cavity, so the retina is re- 
markable for the manner in which it leaves the posterior 
concavity of the eye-cup to approach the centre. ‘The eye 
of Pecten, in fact, presents the interesting peculiarity of the 
approach of the lens and the retina towards the centre, so 
that in P. maximus they almost touch. 

The anterior surface of the retina is convex at the sides 
and concave in the middle, but these convexities and con- 
cavities vary in different species. The different layers of the 
retina will be described from behind forwards, as it will be 
easier to trace the transitions in that way than if described 
from before backwards. They are—1°. Posterior limbs of 
the rods. 2°. Anterior limbs of the rods. 3°. Spindle- 
shaped nucleated rods. 4°. Molecular and nuclear layer. 
5°. Nerves. 

The posterior limbs of the rods stand upon a membrane, 
which runs along the posterior side of the retina; at their 
anterior ends they pierce a very delicate membrane, and pass 
into the anterior limbs of the rods. The anterior limbs are 
about twice as long as the posterior limbs, and are usually 
smaller in diameter, and situated farther apart than the pos- 
terior limbs. That they are circular in section may be 
seen from Pl. II, fig. 7a, which is a drawing of a section 
made at right angles to the eye-stalk. The anterior limbs 
of the rods are sometimes swollen so “as to appear oval ; 
this condition occurs especially in the rods at the side con- 


THE EYE OF PECTEN. 7 


vexities. Fig. 6 represents an isolated rod in this con- 
dition. 

The anterior ends of the rods contract considerably, and 
again expand into spindle-shaped bodies, each of which con- 
tains a nucleus; so that in P.yacobeus, where the retinal 
elements of this region are difficult to distinguish, there may 
be seen a single row of nuclei running from end to end of the 
retina, and following its sinuosities (Plate II, fig. 11 6). 

In some of the rods at the side of the retina a second 
spindle-shaped body follows the first one, as represented in 
the isolated rods in figs. 5, 6, but usually the anterior end 
of the spindle is drawn out into a delicate thread, which 
occasionally possesses nuclear swellings. Finally, this thread 
breaks up into a network, which bears a number of nuclear- 
like bodies at its nodes, and several round molecular bodies 
appear to be caught in its meshes. These bodies are so 
much like the ordinary nuclei of the network that I am in- 
clined to believe that they are, in reality, merely modifica- 
tions of them, and in some way connected with the network 
(fig. 6a). Anteriorly the fibres of the network bend at right 
angles and enter the nerve layer, which covers the anterior 
surface of the retina. ‘This nervous layer will be described 
with the description of the optic nerves. 

The above is a description of the retina as I found it in 
P. maximus, and I believe it holds good for the other mem- 
bers of the genus. The elements of the retina are so much 
larger in this species, and the spaces between the rods and 
network, &c., so much more considerable, that it is a great 
deal easier to investigate ; but I believe careful examination 
of the other species would show that they do not differ from 
this in any important detail. 

The ¢apetum—is placed immediately behind the retina, and 
may help in its support. When fresh,’ the tapetum exhibits 
a display of colours, and it is this membrane which gives 
the eyes their beautiful metallic lustre. When examined 
with a +th-inch obj. it seems to be composed of a great 
number of little black specks separated by a fine yellow 
membrane, but careful examination with a higher power 
shows that it is composed of a great number of fine fibrils 
crossing at right angles. 

The space between the tapetum and the posterior part of 
the eye-cavity is filled with ared fluid pigment. In the fresh 
condition the pigment readily flows on to the slide when the 
eye is pricked, but in sections of the eye which has been 


1 T have one series of sections stained in osmic acid, and mounted in 
Canada balsam, which has retained this display of colours, 


8 SYDNEY J. HICKSON, 


hardened by alcohol or other reagents the pigment adheres 
to the tapetum or posterior wall of the eye-cup. 

Hensen figures a layer of cells in this position, but I have 
never been able to observe anything of the kind; the pigment 
contains no cellular elements at all, nor is there a layer of 
cells lining the cavity which contains the pigment. The 
pigment consists of a number of bright red granules floating 
freely in a colourless fluid. 

The nervous supply—of the eye of Pecten is perhaps the 
most iuteresting of the many peculiarities of this eye. The 
nervous system of Pecten is well described by Duvernoy in 
the paper referred to above. The mantle is supplied by a 
number of branches given off from the principal ganglia. 
These branches all fall into a large nerve, which runs round 
the margin of the mantle, and which Duvernoy calls the 
**circumpalial” nerve. This nerve is figured in section in 
fig. 1, Pl. I, one of the nerves joining this nerve being 
figured at fig. 1, g. This “circumpalial” nerve gives off 
filaments to supply the tentacles and eyes. 

Krohn first gave a drawing of the optic nerve, and described 
it as a single nerve passing off from this trunk, and dividing 
into two branches as it aproaches the eye. Later observers 
have, however, drawn and described two nerves passing off 
from the ‘circumpalial” nerve. My researches have led 
me to believe that Krohn is right, and that such a figure as 
Hensen gives in his paper, representing two main trunks 
passing up to supply the eye is erroneous. Plate II, 
fig. 9, of P. maximus, shows the division of the single nerve 
into its two branches. In fig. 1 the course of the optic nerve, 
before its division into two branches, was carefully drawn 
from one of a complete series of sections, and in none of the 
other sections could I find a trace of any other nerve pro- 
ceeding from the “circumpalial.” The branching of the 
nerve takes place in a plane at right angles to the plane of the 
mantle. When the optic nerve approaches the eye it divides 
into two branches, which may be called the “ retinal nerve” 
and the “complementary nerve.” The former passes up the 
side of the eye cavity, and spreads over the anterior surface 
of the retina; the latter soon loses its sheath, and divides 
up into a number of branches, which supply the tissues 
surrounding the eye. The course which the retinal 
branch takes may be seen in PI. J, fig. 1, and in PI. II, 
figs.8and 9. In figs. 8 and 9, the first section is cut through 
the optic nerve, and shows the manner in which the retinal 
branch runs up the side of the eye-cavity ; the second section 
shows the manner in which the branch bends over on to the 


THE EYE OF PECTEN. Y 


retina and spreads out. ‘The distribution of the comple- 
mentary branch is diagrammatically represented in fig. 17; 
it seems to divide into a number of branches which enve- 
lope the eye-cup, and probably send filaments to the cornea, 
lens, tapetum, and epithelium. 

Comparison of the eyes of the three species, P. maximus, 
P. jacobeus, and P. opercularis.—The eye of P. maximus 
is undoubtedly the most highly developed, the eye of P. 
opercularis is the simplest, whilst P. jacobeus, although 
more like P. opercularis than P. maximus, shows many 
points in which it is intermediate between the two. 

The lens in P. opercularis is separated from the retina by 
a considerable space (Pl. II, fig. 10), and consequently 
the chamber containing the humour is relatively large. In 
P. jacobeus the lens is larger than in P. opercularis, and 
the chamber consequently smaller; and in P. maximus the 
lens is very large, and nearly touches the retina, the cham- 
her of the eye being sometimes very small. A gradation is 
thus observed in the character of this part of the eye in the 
three species. In P. maximus but a small space is filled 
with humour, in P. yacobeus a much larger space is filled 
with it, and in P. opercularis there is a larger space still. 

Again, when the retinas of the three species are compared, 
a similar gradation isfound. The retina of P. opercularis is 
comparatively thin, and the concavity and convexities of its 
anterior surface slight. In P. sacobeus the retina is de- 
cidedly thicker, and the anterior surface is more convex at 
its sides than in P.opercularis ; moreover, it may be noticed 
that the delicate membrane which separates the anterior 
from the posterior limbs of the rods has become bent up in 
the regions corresponding with the anterior convexities of 
the retina. In P. mazimus all these variations become 
much exaggerated. The retina is much thicker than in 
either of the other species; and the side convexities of its 
anterior surface are much bolder (Pl. II, fig. 11,°a, 0, ¢). 
The anterior concavity does not undergo much variation. 

The shape of the membrane separating the anterior and 
posterior limbsof the rods is greatly altered. In P. opercudaris 
this membrane is observed, in section, to stretch from side 
to side without any well-marked curves; in P. jacobeus two 
well-marked curves, corresponding with the anterior con- 
vexities of the retina, are observed; but in P. mazimus these 
curves are converted into two distinct folds, which run up 
into the substance of the retina. The membrane between 
the folds does not sink again as low as it is at the com- 
mencement of the folds, and consequently the central 


10 SYDNEY J. HICKSON. 


part of the retina is raised in the form of a table above 
the level of its sides. This elevation of the central part 
of the retina may be also seen in P. jacobeus, though it 
is not nearly so well marked. The folds which occur in 
P. maximus cause the rods to appear to be given off in a 
pinniform manner at the sides of the retina, and before 
I found the intermediate condition in P. jacobeus I had 
some difficulty in determining the true relationship between 
the retinas of P. maximus and P. opercularis. (Compare 
a, b, c, fig. 11). 

In addition to those just mentioned there are other minor 
points in which the eyes of these species differ from one 
another, such as in the shape of the cells composing the 
lens and in the distribution of the retinal nerve, &c., but 
they are comparatively slight. 

General considerations.—Having thus described, in some 
detaii the anatomy of the various parts which compose 
the eyes of Pecten, I shall, before leaving the subject, point 
out some of their interesting morphological peculiarities. 
It is, in itself, a remarkable thing to find a large and 
variable number of eyes situated on an area at some con- 
siderable distance from any central nerve-ganglion; and, 
when it is remembered that the class and even family (with 
one other exception, e.g. Spondylus) to which the genus 
belongs, possess no organs of vision at all in the adult con- 
dition, it is altogether surprising that they should be of such 
extraordinary complexity as they have proved to be. The 
high structural development that this eye has attained is, 
kowever, not so remarkable as the fact that in many ways 
it differs from the ordinary Invertebrate eye, and resembles 
that of the Vertebrata. 

In the first place, the lens is built up of a large number of 
distinct nucleated cells, which undergo a flat:ening at its 
circumference very similar to that found in the eye of the 
Vertebrata. Whether the lens is developed from the celis 
of the epiblast, as in the Vertebrata, or from the mesoblast, 
must at present be left unsettled, but it will probably be 
found, when the development of the eye is studied, that in 
this respect also it resembles the eyes of the Vertebrata. 
The tapetum, a structure which is of considerable im- 
portance to animals which are nocturnal or aquatic in habit, 
has hitherto been described only in the Vertebrata. That 
Pecten possesses a tapetum as highly developed as any 
found amongst the Vertebrata is anatomically a point of 
considerable interest; but it also indicates to a certain 
extent the physiological capability of the eye. 


THE EYE OF PECTEN. li 


The chief interest, however, lies in the relative positions of 
the optic nerve, the retina, and the pigment. In the eyes of 
the Cephalopods the pigment layer is situated in front of the 
rods, and the nerve-tibres enter the rods from behind. In 
the eyes of the Gasteropoda, the Crustacea, &c., down to the 
simplest form of eye, such as that of the Rotifera, the same 
relationship of these parts holds good. In the Vertebrata, 
however, their relative positions are reversed ; the optic nerve 
pierces the retina, and distributes itself over the front of the 
retina, whilst the pigment is situated behind it. In Pecten 
the relationship of these parts is the same as that in the 
Vertebrata; the nerve passing up the side of the eye-cup 
bends over, and spreads itself over the anterior surface of the 
retina. The pigment also is situated behind the retina. 
Pecten is not, however, the only Invertebrate whose eyes 
are built up on this type. Semper! has recently pointed out 
that on the backs of certain slugs (Onchidium) a number of 
eyes are found, and that in these the nerves pass to the front 
of the retina before being distributed. On account of this 
distribution of the optic nerve he says that they belong to 
the Vertebrate type of eye (typus der Wirbelthieraugen), so 
that two animals are now known, each belonging to a iarge 
and important class of Invertebrata (Gasteropoda and Lamel- 
libranchiata respectively) that possess eyes which are built 
up on this type. ‘The eyes of Pecten are even more deserving 
of the name of Wirbelthieraugen than those of Onchidium, 
for they are much more highly developed, and possess, in 
addition to this relationship of the nerve and retina, other 
Vetebrate peculiarities. The lens is multicellular, a character 
which, altnough not unknown amongst the Invertebrates, is 
much more characteristic of the Vertebrata. The tapetum, 
too, a strueture which doubtfully exists in any other Inverte- 
brata is found in Pecten and some Vertebrates. But, although 
the application of this word Wirbelthieraugen to these eyes 
is convenient for the adult condition, it must be carefully 
remembered that the development of these eyes is essentially 
different from that of the Vertebrate eye. The Vertebrate 
eye is formed in the embryo from a hollow process given off 
from the brain, and the future eye-cup is formed by an in- 
vagination of this process. It is impossible for the eyes of 
Pecten or Onchidium to be formed by any process similar 
to this. Thus, in the young state these eyes are essentially 
different from those of the Vertebrata, and the resemblance 


1 Semper, ‘ Uber sehorgane vom Typus der Wirbelthieraugen auf dem 
Riicken der Schnecken.’? Wiesbaden, 1877. 


12 SYDNEY J, HICKSON, 


in the adult is merely accidental, and by no means due to 
morphological identity. 

Little is known and little can be said concerning the 
function of the eyes of Pecten. The presence of such a 
well-formed tapetum makes it probable that they are 
capable of appreciating very diffused light, and the close 
approximation of the lens to retina makes it exceedingly 
improbable that any image is formed upon the latter. 

A few experiments have been made on the extent of their 
visual power, which make it very doubtful whether they are 
of much value to the animal in avoiding its enemies. The 
most reasonable theory of their function seems to be that, 
when on the ebbing of the tide, a probability arises that 
they will be left high and dry on the shore, they can appre- 
ciate the fact by the growing intensity of the light, and, 
by that peculiar flapping motion of their valves the 
Pectens are so remarkable for, move away into deeper 
water. 

These researches were entirely carried on in the morpho- 
logical laboratory of the University of Cambridge, and my 
best thanks are due to Mr. Balfour for his valuable advice 
and encouragement during the whole course of my researches. 
Owing to his kindness, also, I have been enabled to ex- 
amine some of Semper’s preparations of the eye of 
Onchidium, to which reference has been made in the text. 

Methods.—For a general examination of the eye the best 
method is to harden in alcohol and stain by immersion in 
hematoxylin for twenty-four hours. Of the osmic-acid acid 
preparations the best were obtained by immersion in 
a 1 per cent. solution for fifteen minutes, followed by abso- 
lute alcohol for three or four days. This method is of great 
value for studying the retina and lers. I have also used 
gold chloride for staining the nerves with some success. 
For examining the tapetum the best preparations I have were 
made from some eyes given me by Mr. Haddon, which had 
been treated with picric acid. This reagent seems to have 
dissolved away the red pigment, and consequently left the 
tapetum free from the numerous little red granules which 
generally clingtoit. For examining the isolated rods of the 
retina I have allowed the eyes to remain in a solution of 
chloral hydrate for four or five days. I have then dissected 
out the retina with needles as carefully as possible, and 
poured a drop or two of hematoxylin on totheslide. When 
the retina had been standing in hematoxylin in this manner 
for some hours it was washed with water, teased out with 
fine needles, and mounted in glycerine. 


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DESCRIPTION OF PLATE III, 


Illustrating Mr. Adam Sedgwick’s Paper “On the Early 
Development of the Anterior Part of the Wolffian 
Duct and Body in the Chick, together with some Re- 
marks on the Excretory System of the Vertebrata.” 


List of Reference Letters. 


al., alimentary canal; ao., aorta; c.v., cardinal vein; e. gi., external 
glomerulus; ep., epiblast; hy., hypoblast ; 7. c. m., intermediate cell mass ; 
t. gl., internal glomerulus; &. 4., blastema of Wolffian tubules; me., mesen- 
tery; m.p., muscle plate; zc., notochord; p.c., body cavity; p.f, peri- 
toneal funnel; pv., proto-vertebra; pv.', cell-mass, which later becomes a 
protovertebra; W.d., Wolffian duct; W. ¢., Wolffian tubule. 


Fie. 1.--Section through 10th segment of a chick with ten segments, 
showing origin of Wolffian duct. 


Fic. 2.—Section through 10th segment of a chick with twelve segments. 
Shows second stage in development of Wolffian duct. 


Fies. 3 and 4.—Successive sections through the 10th segment of a chick 
with thirteen segments. Shows further development of Wolffian 
duct. 

Fic. 5.—Section through 10th segment of a chick with fourteen segments, 
showing further development of Wolffian duct and anterior Wolffian 
tubules. 

The above series are all taken through the points where rudimen- 
tary segmental tubes connect the Wolffian duct and peritoneal epi- 
thelium, except Fig. 4, which is through a point between two tubules. 
The object of the series is to trace the continuity in the development 
of the Wolffian duct and anterior tubules, which exists between the 7th 
and 11th segments inclusive. 

Fic. 6.—Section through a chick with twelve segments just behind the 
12th segment. Shows independence of Wolffian duct from peritoneal 
epithelium and intermediate cell mass. 

Fic. 7.—Section through the 13th segment of a chick with thirteen seg- 
ments. Shows how almost at once the Wolffian duct becomes con- 
nected with the intermediate cell mass. The continuity between the 
two structures is not well represented in this figure. 

Fic. 8.—Section through 16th segment of a chick with twenty-one seg- 
ments. Shows separation of Wolffian duct and intermediate cell 
mass, which persists for some time in this region. 

Fics. 9 and 10.—Successive sections through the 15th segment of a chick 
with about twenty-two protovertebre, showing the connection between 
the Wolffian duct and intermediate cell mass. In Fig. 9 the inter- 
mediate cell mass is continuous with the peritoneal epithelium ; in 
Fig. 10 it is separate. 


Se Ot Dee Be 


DESCRIPTION OF PLATE I1I—continued. 


Fig. 12.—Section through a chick late in the third day behind the 16th 
segment, showing the independence of the developing Wolffian tubule 
and the Wolffian duct in this region. 


Fies. 13, 14, and 15.—A series of successive sections through the 13th 
segment of a chick with thirty-one or thirty-two segments, Fig. 13 
being anterior. Show a further stage in the development of a Wolf- 
fian tubule in this region. In Figs. 13 and 14 the tubule is connected 
to the peritoneal epithelium, and a lumen has appeared in it, which 
is continued behind into the part of the tubule separated from the 
peritoneal epithelium, seen in Fig. 15. 


Figs. 16, 17, and 18.—Sections through the 13th or 14th segment of a chick 
with thirty-four or more segments. Show the further development of a 
Wolffian tubule in this region, and the first appearance of the external 
and internal glomeruli. Figs. 16 and 17 are contiguous sections. 
Between Figs. 17 and 18 there is a section not figured. The three 
figures respectively correspond to and are further developments of 
Figs. 13—15. 


Figs. 19, 20, and 21.—Successive sections through the 18th or 14th seg- 
ment of a chick with thirty-six or more segments. Show further de- 
velopment of external glomerulus. 


Fic. 22.—Diagrammatic longitudinal, vertical section, showing the rela- 
tions of the external and internal glomerulus. 


Fie. 23.—Section through the 13th or 14th segment of a chick with thirty- 
three segments, showing the opening of the Wolffian duct near the 
external glomerulus. 


Fig. 24.—Section through the anterior part of the Wolffian body of a 
ae day chick, showing the glomerulus projecting into the Wolffian 
uct. 


Fic. 25.—Section through the hinder region of a tadpole of Rana tem- 
porania, showing the first appearance of the cells from which the 
Wolffian tubules will arise. 


: a i a Poy 
A ak canon in ls Sal i aa Aa lilt ng UNM eae Sel Sj at Mista) emt wat Sa lida agi ae ee 


; dS ia fer 
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Plate Til. 


Plate IL 


Fig 28, 
F.Sodgwisk cOn 


On the Harty DEVELOPMENT of the ANTERIOR Parr of the 
Wotrrran Duct and Bopy im the Cuicx, together with 
some Remarks on the Excretory System of the VERTE- 
BRaTa. By Apam Sepawick, M.A., Fellow of Trinity 
College, Cambridge. With Plate IIT. 


Tue following paper is divided into two parts. The first part 
contains an account of observations on the development of the 
Wolffian duct and anterior Wolffian tubules in the chick, being 
supplementary to my paper on the “Kidney of the Chick.’”! 
The second part is devoted to a discussion of the vertebrate 
excretory system in general. 


I. Early Development of the-Wolfiian Duct and Anterior 
Wolffian Tubules in the Chick. 


The first trace of the Wolffian duct is visible in an embryo 
with eight protovertebre as a slight projection from the inter- 
mediate cell mass towards the epiblast in the region of the 7th 
and 8th protovertebre. The projection also extends back 
behind the region of the protovertebre for a short distance. 
In a chick with nine or ten protovertebre a similar condition 
is found, ze. a projection from the intermediate cell mass 
towards the epiblast in the region of the 7th, 8th, 9th, and 10th 
protovertebre, and for a short distance behind the region of 
the protovertebra. 

In a chick with ten protovertebre the projection is beginning 
to show signs of separation from the imtermediate cell mass at 
certain points. The appearance presented by the rudiment of 
the Wolffian duct in the 10th segment of a chick with ten seg- 
ments is shown in fig. 1. 

In a chick with eleven protovertebre the rudiment of the 
Wolffian duct is still present as a projection from the inter- 
mediate cell mass in the region of the 7th, 8th, 9th, 10th, and 
11th protovertebre ; but behind the region of the protovertebrz 
it has grown back for a short distance between the epiblast aud 
mesoblast as an irregular cord of cells not connected to the 
peritoneal epithelium. A partial separation of the Wolffian 
duct from the intermediate cell mass is now effected in the 
region of the 7th to the 10th protovertebree. This separation 


' “Development of the Kidney in its relation to the Wolffian Body in 
the Chick,” ‘Quart. Journ. Mic. Sci.,’ vol. xx. 


14 ADAM SEDGWICK. 


is not, however, complete; but the Wolffian duct remains connected 
to the peritoneal epithelium at certain intervals by short cords 
of cells. 

In a chick with twelve protovertebre the separation of the 
Wolffian duct from the intermediate cell mass in the region of the 
7th to the 11th protovertebree inclusive is as complete as it ever 
will be, z.e. it has separated for the greater part of its length, but 
remains attached to the peritoneal epithelium at certain points, 
by cords of cells (fig. 2) derived from the cells of the inter- 
mediate cell mass connecting the rudiment of the Wolffian duct 
with the peritoneal epithelium. ‘These cords of cells are the 
commencing Wolffian tubules of the anterior part of the Wolffian 
body, and are more numerous than the segments in which they 
are placed. Behind the region of the protovertebre in a chick 
of this age (twelve protovertebrz), the Wolffian duct has grown 
back as an irregular cord of cells (fig. 6), independent of the 
intermediate cell mass, for a short distance, thus repeating the 
feature of the last and succeeding stages in this particular. In 
the region of the last (12th) protovertebra, however, the cord of 
cells constituting the Wolffian duct at this stage is now con- 
tinuous with the intermediate cell mass at certain intervals. 
Comparing the sections through the 12th segment of this stage 
with those just behind the 11th protovertebra of the previous 
stage, it is seen that the Wolffian duct has enlarged, and by a 
downgrowth of cells from it, with which probably is connected 
an upgrowth from the intermediate cell mass, has become in 
certain places connected with the intermediate cell mass. These 
secondary connections constitute the commencing tubules of this 
part of the Wolffian body. 

In a chick with thirteen protovertebree an advance precisely 
similar to that characterising the previous stage has taken place, 
z.e. the Wolffian duct has become connected with the inter- 
mediate cell mass in the 13th segment (fig. 7), and behind 
this point is free from adjacent structures. 

In a chick with fourteen or fifteen protovertebre the process 
of development remains the same. So that in a chick with 
fifteen segments the following is the condition of the Wolffian 
duct :—It extends from the 7th to the 15th segment as a solid 
cord of cells, connected at intervals with the peritoneal epithe- 
lium by the commencing Wolffian tubules; behind the 15th 
segment it extends for a short distance asa free cord. The further 
development differs from that just recorded in this important 
particular; the duct does not become connected with the inter- 
mediate cell mass of the newly-formed last segment, but remains 
separate for a considerable interval of time (till towards the end 
of the third day) from it. In other words, the formation of 


WOLFFIAN DUCT AND BODY IN THE CHICK. 15 


the Wolffian tubules and their connection with the Wolffian 
duct is deferred behind the 15th segment. 

To sum up the developmental changes above recorded, the 
Wolffian duct arises as a continuous ridge of cells projecting 
from the intermediate cell mass towards the epiblast in the 
region of the 7th to 11th protovertebre inclusive. ‘This ridge 
separates from the intermediate cell mass from before backwards, 
remaining, however, connected with it at intervals by the rudi- 
mentary Wolffian tubules. Meanwhile, from the hind end of it 
there grows back a cord of cells independent at first of the 
adjacent structures, but immediately on the formation of the 
hinder segments becoming connected with the intermediate 
cell mass of each segment in turn. ‘This happens as far back 
as the 15th segment; behind this point it grows back as a 
solid cord, which does not become connected with the inter- 
mediate cell mass until the tubules of the Wolffian body have 
made considerable advance in their development. 

Figs. 1—7 are meant to illustrate the above method of develop- 
ment. Figs. 1—5 are from the 10th segment of chicks, with 
ten, twelve, thirteen, and fourteen protovertebre respectively. 
They are all taken through points where the Wolffian duct remains 
attached to the peritoneal epithelium, 7.e. through a rudimentary 
tubule, excepting fig. 4, which is from a section close to fig. 3, 
and shows the condition of things in one of the intervals between 
the points of continuity. 

Fig. 6 is taken from a section just behind the last segment of 
a chick with twelve segments, and shows the complete inde- 
pendence of the Wolffian duct. 

Fig. 7 is from the 13th segment of a chick with thirteen seg- 
ments, 7.e. from the same region as fig. 6, and it shows the con- 
nection which has become established between the Wolfhan ~ 
duct and the intermediate cell mass by a mutual growth of these 
structures. 

Fig. 8 is from the 16th segment of a chick with twenty-two 
protovertebre, and is illustrative of the fact derived from an 
inspection of all the sections of the segment, that the Wolffian 
duct is independent of the peritoneal epithelium. From the 
15th segment the Wolffian duct grows back independently to 
the cloaca, into which it eventually opens, and a lumen appears 
in it from before backwards. 

In fig. 11, taken from a chick at the end of the third day, it 
is still distinct from the now considerably developed Wolffian 
tubule (w.72.). 

_ For purposes of description I shall divide the Wolffian body 
into three regions—(1) ‘The part found within the limits of the 
7th—11th segments inclusive ; (2) the part found within the 


16 ADAM SEDGWICK. 


12th—15th segments inclusive ; (3) that found behind the 15th 
segment. 

In a previous paper! I have described at some length the 
early development of the Wolffian body behind the 16th segment, 
and I have there shown that that part may be divided into two 
parts, each characterised by a peculiarity in the early develop- 
ment. In this paper I shall make but little reference to the 
development of the Wolffian body in this region, confining 
myself almost entirely to that part lying within the area of the 
7th to the 15th segments inclusive. 


Development of Wolffian Tubules im region of Tth—11th 
Segments. 


The Wolffian tubules and Wolffian duct in this region attain 
but a slight development. They may almost be said to have 
reached their highest point at the stage with fourteen proto- 
vertebra, the only difference in later stages being the develop- 
ment of a lumen in them. The lumen in the tubule may 
acquire an opening into the Wolffian duct in some cases. In 
this case the string of cells seen in fig. 5 becomes very short, 
and the Wolffian duct appears as a narrow groove in the peri- 
toneal epithelium. This state of things is usually found in 
chicks with from nineteen to thirty-two protovertebre. 

The Wolffian duct in this region exhibits great variations in 
calibre, and in later stages parts of it appear to atrophy, and 
isolated portions are found connected with rudimentary tubules. 
An enlarged section of the Wolffian duct in front is nearly 
always found as Gasser? has described. The duct and tubules 
in this region appear entirely to atrophy in chicks with more than 
thirty-five protovertebre. 

I have not thought it worth while to preserve figures of the 
duct and tubules in this region of the Wolffian body after their 
first appearance, as the arrangement just described may be easily 
observed in sections of an embryo chick of the third day. 

The interest in the development of this region lies in the fact 
of the continuity of development of the Wolffian tubules and 
Wolffian duct. It has always appeared to me astonishing that 
the Wolffian duct developed as a continuous ridge from the inter- 
mediate cell mass, which, from our knowledge of Elasmobranch 
development, may be called the peritoneal epithelium, should 
entirely separate from it and then secondarily become connected 
with it by the tubules of the Wolffian body. My investigations, 
which have been made with some care on a large number of 


1 Loc. cit. 2 Loc. cit. 


WOLFFIAN DUCT AND BODY IN THE CHICK, 17 


chicks of all ages from nine to thirty protovertebre, have 
entirely convinced me that the usual statements on this point are 
not true, and show to my mind most conclusively that the duct 
and tubules of the Wolffian body in the region in question do 
develop in continuity, precisely as do the duct and peritoneal 
openings of the head-kidney in most Ichthyopsidan types. 

The number of rudimentary tubules in each segment of this 
region I have not determined precisely. They occur as often 
as not between the segments, and there seems to be about two for 
each segment. In the seventh segment I have never seen more 
than one. 

Before proceeding to give an account of the further develop- 
ment in the next region, I will briefly refer to the points in 
which my observations differ from those of previous observers 
on the development of the Wolffian duct. 

Gasser’s account! of the development of the Wolffian duct is 
the most recent and exact. In his valuable paper will be found 
a complete account of the literature of the subject, to which I 
need not further refer. 

“The first trace of it which he finds is visible in an embryo 
with eight protovertebre as a slight projection from the inter- 
mediate cell mass towards the epiblast in the region of the three 
hindermost protovertebree. In the next stage with eleven pro- 
tovertebree, the solid rudiment of the duct extends from the 5th 
to the llth protovertebre ; from the 8th to the llth 
protovertebrze it lies between the mesoblast and epiblast, and is 
quite distinct from both, and Dr. Gasser distinctly states that 
in its growth backwards from the 8th protovertebre the 
Wolffian duct never comes into continuity with the adjacent 
layers. In the region of the 5th protovertebra, where the 
duct, &c., was originally continuous with the mesoblast, it has 
now become free, but is still attached in the region of the 6th 
to the 8th. In an embryo with fourteen protovertebre the 
duct extends from the 4th to the 14th, and is now free between 
epiblast and mesoblast for its whole extent.” 

The points in which the preceding account differs from that 
of Dr. Gasser’s briefly are: 

1. The position of the continuous ridge of the Wolffian duct. 

2. The subsequent complete isolation of the duct in the 
region of the ridge. 

3. The independence of the backward growth of the duct in 
the 12th to the 15th segment. . 

I have never seen any trace of the Wolffian duct in front of 
the 7th segment, and in all the chicks [ have examined I find 


1 © Arch. fiir Mic. Anat.,’ vol. xiv. 


18 ~ ADAM SEDGWICK. 


that the continuous ridge extends from the 7th to the 11th 
segments. 

With regard to Gasser’s statement of the complete isolation 
of the duct in the anterior region from the intermediate cell 
mass, I can only say that my observations point to an entirely 
different conclusion. 

Thirdly, I differ with him in his statement that the duct in 
the growth back from the attached extremity does not come 
into relation with adjacent structures. 

As stated above, it seems to me that for the space of four 
segments the small cord of cells which grows back from the hind 
end of the ridge, does almost immediately become connected 
with the intermediate ceil mass. 


Development of the Wolffian Duct and Body from the 12th—15th 
Segment. ) 


I now pass to the most interesting point which has turned 
up in my investigations on the excretory system of the chick. | 

In a paper by Mr. Balfour and myself,' describing the develop- 
ment of what we believed to be a rudimentary head-kidney in 
the chick, we drew attention to a structure which so closely re- 
sembled the glomerulus* of the head-kidney of the Ichthyopsida 
that we identified it as an homologous structure. 

Gasser? has also independently discovered and similarly iden- 
tified this structure. 

In the paper just referred to no attempt was made to trace the 
development of this glomerulus, but it was merely described as 
it appeared at the time of its greatest development. 

The following description is taken from that paper : 

“Tn the chick the glomerulus is paired, and consists of a 
vascular outgrowth or ridge projecting into the body cavity on 
each side at the root of the mesentery. It extends from the 
anterior end of the Wolffian body to the point where the fore- 
most opening of the head-kidney commences. We have found 
it at a period slightly earlier than that of the first development 
of the head-kidney....In the interior of this body is seen a 
stroma with numerous vascular channels and blood-corpuscles, 
and a vascular connection is apparently becoming established, if 
it is not so already, between the glomerulus and the aorta. The 
stalk connecting the glomerulus with the attachment of the 

1 *On the Existence of a Head-Kidney in the Embryo Chick: Studies 
from the Morphological Laboratory in the University of Cambridge,’ Part 1, 
1880, and ‘ Quart. Journ. of Micr. Science,’ vol. xix. 

2 | have already given a preliminary account of the development of this 
structure in the ‘ Proc. Cambridge Phil. Soc.,’? May 3, 1880. 


3 * Sitzungsberichte der Gesellschaft zur Bedford d, gesam. Naturwiss.,’ 
No. 5, 1879. ' 


WOLFFIAN DUCT AND BODY IN THE CHICK. 19 


mesentery varies in thickness in different sections, but we believe 
that the glomerulus is continued unbroken throughout the very 
considerable region through which it extends. This point is, 
however, difficult to make sure of, owing to the facility with 
which the glomerulus breaks away. At the stage we are 
describing no true Malpighian bodies are present in the part of 
the Wolffian body on the same level with the anterior end of the 
glomerulus, but the Wolffian body merely consists of the Wolffian 
duct. At the level of the posterior part of the glomerulus this 
is no longer the case, but here a regular series of primary Mal- 
pighian bodies is present, and the glomerulus of the head-kidney 
may frequently be seen in the same section as a Malpighian 
body. In most sections the two bodies appear quite discon- 
nected, but in those sections in which the glomerulus of the 
Malpighian body comes into view it is seen to be derived from 
the same formation as the glomerulus of the head-kidney.” 

The point which is left in doubt in the above description, 
viz. as to whether the glomerulus constitutes a continuous 
structure, is at once decided by a study of its development. 

I may here state that it is not a continuous structure, but 
consists of a series of external glomeruli, each of which corre- 
sponds and is continuous with the glomeruli of the Malpighian 
bodies found in this part of the trunk. 

The first development of the Wolffian tubules in the region 
under consideration has already been described. They appear 
as outgrowths from the Wolffian duct meeting outgrowths from 
the intermediate cell mass immediately on the formation of the 
segment in which they are placed; so that in a chick with fifteen 
protovertebree the Wolffian duct is connected with the inter- 
mediate cell mass by a certain number of cell cords in the 12th, 
13th, 14th, and 15th segments. 

The duct and cords, which have at first rather an irregular 
outline, soon become well-defined compact structures. 

Fig. 12, taken from the 12th segment of an embryo with 
twenty-two segments, represents the condition of things at this 
age. 

The Wolffian tubules in this region are derived from two 
distinct structures—(1) the outgrowth from the Wolffian duct ; 
(2) part of the intermediate cell mass. 

The intermediate cell mass is at first continuous with the 
peritoneal epithelium in every section; but, as described in a 
previous paper, this connection soon becomes lost at certain 
points (fig. 9), and maintained at others (fig. 10). Figs. 9 and 
10 are contiguous sections through the 15th segment of a chick 
with twenty-two segments, showing this point. At these :oints, 
where the continuity is retained, a peritoneal funnel is subse- 


20 ADAM SEDGWICK. 


quently formed by the development of a lumen extending from 
the body cavity into the intermediate cell mass. 

The features of the stage of development now reached are well 
known; it is that of the S-shaped cords of cells which have been 
so often described. In the adjoining woodcut is represented part 
of one of these S-shaped strings, showing clearly the above 


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Transverse Section through the Trunk of a Duck Embryo with about 
twenty-four Mesoblastic Somites. 
am. amnion; so. somatopleure ; sp. splanchnopleure ; wd. Wolffian duct ; 
st. segmental tube; ca.v. cardinal vein; ms. muscle-plate; sp.g. spinal 
ganglion; sp.c. spinal cord; ch. notochord ; ao. aorta; Ay. hypoblast. 


features of a tubule, &c., viz.—(1) the Wolffian duct, in which a 
lumen has appeared; (2) the outgrowth from it to the inter- 
mediate cell mass forming the upper limb of the S; (38) the 


———————— 


WOLFFIAN DUCT AND BODY IN THE CHICK. 21 


intermediate cell mass with the commencing lumen from the body 
cavity. 

In the next section the intermediate cell mass is not 
connected to the peritoneal epithelium. 

Inchicks of gradually increasing number of protovertebre this 
cavity in the intermediate cell mass gradually becomes more 
marked (figs. 13, 14), and extends into that part of it imme- 
diately behind the peritoneal connection (fig. 15). 

Figs. 13, 14, and 15 are three successive sections through the 
13th segment of a chick with about thirty segments, showing 
the features of a tubule at this stage. 

The Wolffian duct is connected with the lower end of the in- 
termediate cell mass in all the three sections. A distinct lumen 
has appeared in the intermediate cell mass which opens into the 
body cavity in front (figs. 13 and 14), but is separate from the 
body cavity in the hindermost section (fig. 15). 

Comparing these figures with figs. 9 and 10 it is seen that 
fig. 13 or 14 corresponds to fig. 9 in the fact of the continuity 
between the intermediate cell mass and peritoneal epithelium ; 
while fig. 15 corresponds to fig. 10, in both the continuity 
having been lost. The difference between them consists in the 
presence of a distinct lumen in the older series, opening into 
the body cavity, and continued behind into the part of the 
intermediate cell mass which has separated from the peritoneal 
epithelium. ‘This part, marked 7. e. m. in fig. 15, will in the 
next stage become converted into that part of the tubule in 
which a Malpighian body is developed, while the anterior part, 
which is open to the body cavity, will widen out considerably, 
and give rise to a wide peritoneal funnel, 

In fig. 11 is represented a section through a developing 
Wolffian tubule in the hinder part of the Wolffian body. The 
tubule (w. #1.) in this section precisely resembles the part of the 
tubule (2. ce. m.) represented in fig. 15. Supposing the anterior 
part of w. 1. were open to the body cavity it would almost be a 
repetition of the anterior tubule, save in the fact that it is not yet 
united to the Wolffian duct. But the hinder tubule (fig. 11) 
does not develop until after the intermediate cell mass has sepa- 
rated from the peritoneal epithelium, 2. e¢. subsequent to the 
obliteration of the rudiment of the peritoneal funnel. 

Not only do the Wolffian tubules in the region of the 12th 
to 15th segments develop a lumen while still continuous with the 
peritoneal epithelium, but further, a glomerulus appears in them 
while still open to the body cavity ; and this glomerulus not 
only appears in the hinder part of the tubule (fig. 15) which 
has separated from the peritoneal epithelium, but also in the 
anterior part (figs. 13 and 14) where it is open to the body 


22 ADAM SEDGWICK. 


cavity. This is at once clear on inspection of figs. 16, 17, 18. 
These figures are taken from the 13th segment of a chick with 
thirty-four protovertebre. There was a section not figured 
between fig. 17 and 18, otherwise the sections are successive, 
fig. 16 being the anterior. ; 

In fig. 16 is seen the commencement of the peritoneal funnel 
as a bay lying between the Wolffian duct and mesentery. 

In fig. 17, a glomerulus (9/.) has appeared projecting into this 
bay. In the next section, not figured, the bay was almost 
closed up by an approximation of its edges, while in fig. 18 the 
bay is completely shut off from the body cavity, and we have a 
section of a true Malpighian body with its contained glomerulus. 

Fig. 18 clearly corresponds to fig. 15 of the previous stage, 
while fig. 17 corresponds to fig. 14, the difference being that a 
distinct cellular projection (g/.) has appeared at the point where 
the projection of cells from the Wolffian duct joins the inter- 
mediate cell mass. 

I have given a diagram (fig. 22) representing an ideal longi- 
tudinal dorso-ventral section through two of these Wolffian 
tubules at this stage. This diagram has been made from a study 
of many embryos showing the development of the external 
glomerulus. 

The open peritoneal funnel is represented at p. f., the arrow 
pointing into it. Through it is projecting the anterior part of 
the glomerulus (g/.), that part which I shall call the external 
glomerulus. A transverse section through this part would 
give the appearance represented in fig. 17. 

Into the closed hinder part of the tubule (2d.) is projecting 
the hinder part of the glomerulus (2. g/.), which I shall call the 
internal glomerulus. It was not possible to represent satis- 
factorily in this diagram the Wolffian duct, which, obviously 
from its position in transverse section, would not be seen in a 
longitudinal section passing through the attachment of the 
glomerulus. 

In fig. 23 is represented somewhat diagrammatically a trans- 
verse section through a chick with thirty-three protovertebre, 
i.e. from a slightly younger embyro than that from which 
figs. 16—18 were taken, in which the cord of cells connecting 
the Wolffian duct with the cavity of the glomerulus had acquired 
a distinct lumen, the cavity of the Wolffian duct being here 
distinctly continuous with that of the bay in which is placed 
the rudimentary external glomerulus, and so with the body 
cavity. At subsequent stages this part of the tubule appears to 
persist, but only in a rudimentary fashion. 

The next stage which I propose to describe was found in a 


ee 


WOLFFIAN DUCT AND BODY IN THE CHICK, 20 


ehick in which thirty-six protovertebre could be counted, but 
possibly there were more. 

The glomerulus has grown immensely (figs. 19, 20, 21), and 
has now acquired the peculiar histological features which 
characterise it at the time of its greatest development, and which 
have already been described in a former paper. 

Anteriorly the bay has widened out considerably (fig. 19), 
and the glomerulus (e. g/.) projects directly into the body 
cavity. Posteriorly the bay remains deep (figs. 20, 21), and the 
glomerulus almost completely fills it and projects beyond it into 
the body cavity. In sections behind fig. 21 there was seen a 
fairly well-developed internal glomerulus. 

The edges of the bay are gathering round the glomerulus pre- 
paratory to fusing with it, and so closing up the peritoneal 
funnel and dividing the glomerulus completely into two parts, 
the internal vascular tissues of which, however, are continuous. 

In this stage the epithelial covering of the external glomerulus 
(e. gl.) was distinctly, as in the previous stage, continued behind 
directly into that covering the posterior internal glomerulus. 

When, however, the peritoneal funnel closes by the comple- 
tion of the process commencing in figs. 20 and 21, this epithelial 
continuity is lost, and we have the final stage of the glomerulus, 
the last which I have observed, in which the separation above 
described is complete, so that in this stage, which is that of the 
greatest development of the external glomerulus, and corre- 
sponds with the commencing formation of the head-kidney, the 
glomerulus belonging to one tubule is divided into three parts. 

(1) An anterior! part projecting into the body cavity. ‘This 
corresponds to a further development of fig. 19. 

(2) A middle part, continuous with (1), also projecting 
freely into the body cavity, but also connected by vascular 
structures with an internal glomerulus. This part is figured in 
fig. 26, and corresponds to a further development of the part 
from which fig. 20 and 21 were taken. 

(3) A posterior part, in which there is no external glomerulus, 
but merely an internal one belonging to a true Malpighian 
body of the mesonephros, which I have not thought it neces- 
sary to figure in this or the previous stage. It is a further 
development of fig. 18. This stage, which may be observed 
about the middle of the fourth day of incubation, brings to a 
close my observations on this extraordinary structure. It appears 
that in the chick the stage just described is that of the greatest 
development of the external glomerulus. In the duck, however, 


\ Fig. KE, Pl. II, in the paper on the ‘ Head-Kidney of the Chick: 
Studies from the Morphological Laboratory in the University of Cambridge,’ 
Part 1, 1880, and ‘ Quart. Journ. Mic. Sci.,’ vol. xix. 


24 ADAM SEDGWICK. 


I have often met with it even larger and more developed, and 
it appears to me after its separation from the internal glome- 
rulus to get an independent growth, and while the latter is 
undergoing atrophy to become larger and extend itself posteriorly, 
so as almost to overlap the external glomerulus of the next 
tubule. 

With regard to the number of the external glomeruli in the 
chick and the exact limits of their occurrence, the following -is 
briefly what I have been able to make out in a chick with thirty 
protovertebree : 

In the 1lth segment there are two rudimentary tubules 
running from the Wolffian duct to the peritoneal epithelium. 
At the point of attachment of these there 1s a small rudiment of 
the external glomerulus, visible for only one section in each case. 

In the 12th segment there is at the beginning a Wolffian 
tubule and a well-marked external glomerulus extending through 
three sections. At the hind end of the 12th segment and 
beginning of the 13th there is an external glomerulus for three 
sections continued into part of the segmental tube behind, in 
which an internal glomerulus will subsequently be developed. 

In the 13th segment there is an external glomerulus for three 
sections. 

In the 14th segment there are two segmental tubes with 
developing external glomeruli. 

In the 15th segment no external glomeruli appear to be 
developed, the segmental tubes being already separated from 
the peritoneal epithelium. 

In later stages only the three or four hindermost of the above 
external glomeruli appear to develop further. The anterior 
glomeruli soon atrophy with the adjoining tubules and duct. 

In the duck a much greater number become developed, 
and they may be seen in the anterior segments after their 
respective tubules have entirely atrophied. 

The bearing of the developmental processes above recorded on 
any hypothesis as to the phylogenetic history of the vertebrate 
excretory system I propose to examine in the second part of this 
paper (pp. 41—43; 47). 


Parr II. A Discussion of the Vertebrate Exeretory System im 
General. 


The most peculiar feature of the excretory system of the 
vertebrata is the presence of three more or less distinct parts, the 
pronephros, the mesonephros, and the metanephros or kidney 
proper. In the following pages my object will be to explain the 
relation of these parts, more especially those of the pronephros 


WOLFFIAN DUCT AND BODY IN THE CHICK, 25 


and mesonephros, and to show that they have arisen as differen- 
tiations of a primitively uniform structure. 

For this purpose it is necessary briefly to recapitulate the 
more important features in the development which have a 
bearing on my argument. 


Segmental Duct and Pronephros. 


The first part of the excretory system to make its appearance 
is always a duct. This duct has received various names, but its 
homology in different forms is undisputed. I shall call it the 
segmental duct. 

In the chick the segmental duct is commonly known as the 
Wolffian duct. 

All the Ichthyopsida whose development is known, with the 
exception of Hlasmobranchs, possess a structure called the head- 
kidney or pronephros. The pronephros when present always 
develops in continuity with the anterior end of the segmental 
duct. 

In the Amphibian the segmental duct arises as a groove of 
the parietal peritoneum, just ventral to the place where the bod 
cavity is connected with the cavities of the muscle plates. This 
groove, which arises first of all anteriorly just behind the 
branchial region, is continued for a certain distance backward. 
It soon, however, becomes constricted into a canal lying between 
the ectoderm and parietal peritoneum. This constriction has 
been described as taking place in the following manner :—It 
first appears in the middle region of the groove, giving rise to a 
canal opening into the body cavity in front and behind. It 
then is continued backwards until the groove is completely con- 
verted into a canal behind, which soon acquires an opening into 
the cloaca. Anteriorly the wide opening meanwhile is divided 
up into two,! three,” or four® openings, according to the species. 

The canal immediately behind the last of these openings 
becomes coiled and placed on the same level but ventral to the 
openings. ‘The part of the body cavity into which the openings 
of the segmental duct pass widens out, a vascular projection— 
the glomerulus—from the dorsal inner wall is formed, extending 
uninterruptedly from opposite the anterior opening of the seg- 
mental duct to as far back as the posterior. ‘The dilated section 
of the body cavity in which the glomerulus lies, and into which 
the segmental duct opens, is partially separated from the rest of 
the body cavity. The whole structure, including openings of 
duct, ventral coiled part of duct, glomerulus, and dilated part of 
body cavity, is known as the pronephros, The number of open- 

' Urodela. ? Anura, > Cecilia. 


3 


26 ADAM SEDGWICK. 


ings from the segmental duct into the body cavity corresponds 
with the number of segments through which the pronephros 
extends.1 

With its excretory system in this condition the young 
Amphibian is hatched. Fundamentally the head-kidney retains 
the above structure, increasing only in size until it begins to 
atrophy, an occurrence which takes place on the development 
of the mesonephros. 

This method of development of the segmental duct and pro- 
nephros is fundamentally repeated in other animals which possess 
a pronephros. 

About the marsipobranch development very little is known. 
Fiirbringer (loc. cit.), quoting W. Miller and his own observa- 
tions, makes the following statements for Petromyzon :—In the 
earliest stage which has been observed there was present at 
about the level of the heart a groove in the parietal peritoneum, 
which leads behind into a duct, which eventually, by a backward 
growth, reaches the cloaca and opens into it. The anterior 
groove or opening of the duct soon becomes divided up into 
four openings. 

In the young Ammocetes there is present a pronephros made 
up of a complicated coiled duct and four or five openings into 
the body cavity, opposite which is placed a vascular glomerulus ; 
the whole structure extends over four or five segments.” The pro- 
nephros atrophies in the adult. 

In Myxine nothing is known of the development, but in, the 
adult a pronephros has been described, which, however, is not 
functional in old individuals (adult ?), as in them it has lost its 
connection with the backward continuation of the segmental 
duct. 

It? consists of the segmental duct, which gives off dorsally a 
number of diverticula, in which are found glomeruli, and ven- 
trally a number of coiled canals, which open apparently into 
the pericardial cavity. 

The fully-formed pronephros of Petromyzon then resembles 
in structure very closely that of Amphibia, while the pronephros 
of Myxine differs in certain important points. | 

The Teleostei possess a pronephros, which persists as a large 
organ in the adult. It develops in connection with the seg- 


1 Viirbringer, ‘ Morph. Jahrbuch,’ Bd. 3, p. 5. 

2 Scott, in a recent paper (‘ Morph. Jahrbuch,’ vol. viii), states that the 
segmental duct in Petromyzon, develops as a solid cord of cells from the 
somatic mesoblast, which subsequently becomes hollow. The peritoneal 
openings of the head-kidney are developed as outgrowths from the anterior 
end of this duct to the body cavity. 

3 * Jenaische Zeitschrift,’ vol. vil, 1873. 


WOLFFIAN DUCT AND BODY IN THE CHICK. at 


mental duct precisely as does the pronephros in Amphibia. The 
only difference between the two is that in Teleostei the segmental 
duct has never more than one anterior opening, and the part of 
the body cavity into which it opens, and in which the glomerulus 
lies, is completely constricted off from the rest of the body cavity, 
and comes to resemble exactly an enormous Malpighian body." 

I may here sum up the common features characterising the 
ontogeny of the pronephros and its duct (segmental duct) in 
all the forms of the Ichthyopsida in which the development is at 
all known: 

1. The segmental duct arises first as a ridge from the 
parietal peritoneum. This ridge usually contains a diverticulum 
from the body cavity, and is continuously constricted off to form 
a duct.” 

2. Except anteriorly, where the constriction only takes place 
at intervals, leaving the openings of the pronephros (except in 
Teleostei, where there is only one opening). 

3. These openings correspond in number with the segments 
which the pronephros occupies.? 

4, A vascular structure, called glomerulus, is formed, pro- 
jecting on each side of the aorta into a specialised dilatation of 
the anterior part of the body cavity. Myxine forms a peculiar 
exception to this otherwise universal fact. 

5. This dilated part of the body cavity may become partially 
or completely separated off to form a capsule, into which the 
glomerulus projects and the anterior end of the segmental duct 
opens. 

"6. The pronephros in all those Ichthyopsida in which it is 
found attais a functional development, but is usually only 
active during a period intervening between the hatching and the 
attainment of full maturity, 2. e. it only functions in the larva. 

In Elasmobranchs, which do not, so far as is known, possess 
a pronephros, the segmental duct arises as a solid ridge from the 
somatic layer of the intermediate cell mass in the anterior region 
of the trunk. From this ridge there grows back a column of 
cells to the cloaca. On the development of a lumen the seg- 
mental duct, with its peritoneal opening, is established. The 
duct develops quite independently of adjacent structure behind 


1 There is a functional head-kidney in adult Ganoids. It appears to 
be formed on the Teleostean type (vide Balfour, ‘Comp., Embryology, 
vol. 2, p. 51). 

2 In Petromyzon, Scott (see note, p. 445) states that this duct arises as 
asolid rod of cells, which secondarily becomes connected with the body- 
cavity epithelium, to form the pronephric funnels. This account, in my 
opinion, needs confirmation. 

$< Pirb.,’ p. 5, p. 42. 


28 ADAM SEDGWICK. 


the point of. its original attachment, and does not unite with 
the segmental tubes till considerably after its first development. 

The difference in the development of the segmental duct in 
the forms possessing a pronephros and in Elasmobranchs is only 
one of degree. 

In both cases it at first arises as a projection, either solid or 
containing a diverticulum from the body cavity, from the 
parietal peritoneum just ventral to the muscle plates ; but in the 
one case this groove has a greater longitudinal extension than 
in the other. In all probability the hinder part of the seg- 
mental duct is in all cases formed by an independent growth 
from the hind end of this groove. 

Amongst the Amniota the chick is the type in which the 
development of the segmental duct has been most carefully 
examined. 

In the chick it arises as in Amphibia as a projection (solid in 
the chick) from the parietal mesoderm just ventral to the 
muscle plates; and the extent of the ridge is the space occupied 
by five segments. 

This ridge is constricted off at intervals from the intermediate 
cell mass, but remains attached at certain points. The hind end 
of the duct is formed by a growth back from the hind end of 
this ridge, which takes place independently of adjacent 
structures. 

The question now presents itself: are these structures at the 
anterior end of the segmental duct in the chick, which so closely 
resemble in development the openings of the Ichthyopsidan 
head-kidney, homologous with that head-kidney ? 

To a consideration of this question I shall return. 


Mesonephros. 


The mesonephros obtains a large development in all the 
groups of the Vertebrata; but it does not persist as an excre- 
tory organ in the adult of the Amniota. 

It develops in three very markedly distinct ways. 

The first of these characterises the Elasmobranchii. 

The second the Amphibia, Teleostei, Ganoidei, Marsipo- 
branchii. 

The third the Amniota. 


The Development of Mesonephros in Elasmobranchi. 


The segmental tubes of Elasmobranchii were originally de- 
scribed by Balfour as arising as solid diverticula of the peritoneal 
epithelium. An examination of Balfour’s specimens led me, 
however, to conclude that they originated as specialised parts 
of the body cavity, viz. from the canals in the intermediate 


WOLFFIAN DUCT AND BODY IN THE CHICK, 29 


cell mass which connect the muscle plate cavities with the general 
body cavity; and Balfour has now given his adherence to this 
view (‘Comp. Embryology,’ vol. 2, p. 570). 

These canals having lost their connection with the body cavity 
of the muscle plates acquire an opening into the segmental duct, 
and differentiate! into the typical Wolffian tubules. The connec- 
tion with the general body cavity may or may not be retained in the 
adult. The secondary tubules develop as outgrowths from that 
part of the primary tubules, which will give rise to a Mal- 
pighian capsule. These outgrowths grow forward and even- 
tually acquire an opening into the terminal portion of the 
tubule of the segment in front. Later they loose their connec- 
tion with the Malpighian capsules, though a rudiment of this is 
sometimes retained as a solid cord of cells. 

The method of development of the secondary, tertiary, &c., 
tubules has not been followed. 

The primary tubules open into the segmental duct very shortly 
after the latter has acquired an opening into the cloaca. 

The formation of the Malpighian bodies and the outgrowths 
from them to form secondary tubules occur later. 

For a full account of the development of the mesonephros 
in Hlasmobranchs I must refer to the works of Balfour and 
Semper, to whom we owe the whole of our knowledge. 


Development of the Mesonephros in the remainder of the 
Ichthyopsida. 


As a type of this development I will take an Amphibian, 
Salamandra, in which animal it has been more completely 
elucidated by Fiirbringer than in any other.? 

Fiirbringer describes the formation of the mesonephros as 
taking place entirely during larval life; no trace of the gland 
being seen in the newly hatched larva. It arises as a series of 
ingrowths of the peritoneal epithelium, which soon become 
separate from the latter. The primary tubules are hollowed out 
in the cell masses so formed independently both of the body 
cavity and segmental duct (Wolffian duct), but subsequently 
they acquire an opening into both. 

The secondary tubules arise in a blastema, the origin of 
which is not clear, but is apparently derived from the just 
mentioned serial ingrowths. They acquire an opening into 
the collecting part of the primary tubule and into the body 
cavity. The remaining dorsal tubules have an equally obscure 
origin. 

' ¢Elasmobranch Fishes,’ p. 260 e¢. seg. — 
* Loe. cit. 


30 ADAM SEDGWICK. 


As the mesonephros becomes more developed the pronephros 
retrogrades, and is eventually entirely, as far as its function is 
concerned, replaced by the former. 

The development of the mesonephros in Teleostei, Marsi- 
pobranchii, Ganoidei, is similary described as taking place in the 
free young (larva) from strings of cells derived from the peri- 
toneal epithelium. In Marsipobranchii as in Amphibia the young 
are hatched with a functional pronephros, and no trace of the 
mesonephros; but the former is, in the further growth of the 
young animal, gradually replaced functionally by the latter, and 
more or less retrogrades. In the Teleostei, however, and 
Ganoidel, it persists with the mesonephros as an important 
functional organ in the adult. In some Teleostei the pronephros 
is the only functional aduit kidney, the mesonephros not being 
developed. 

I have made some observations on the development of the 
mesonephros in the Frog (Rana temporaria), Salmon and Stur- 
geon, and my observations lead me very strongly to doubt whether 
Fiirbringer and other observers are right in describing the origin 
of the cells which give rise to the mesonephros as actual 
ingrowths from the peritoneal epithelium. 

In the case of the Frog this is certainly not the case. In 
fig. 25 is represented a section through a Tadpole of 11 mm., 
showing the first trace of the cells (K 8B) from which the Wolffian 
tubules arise. At their first appearance they are independent 
of the peritoneum, and only secondarily become connected 
with it. Fiirbringer figures from the Salamander a section 
in support of his statement; I have also seen such appearances 
in the Tadpole, but in this animal these strings are only found 
in that part of the animal in which, I am confidently able to state, 
no Wolffian tubules are ever developed. I have examined and 
compared segment with segment of Tadpoles of various ages, 
and have never found these strings of cells developing ito 
Wolffian tubules. ‘The cell strings appear to me to arise from a 
blastema of cells developed zu si¢t@ becoming connected with the 
peritoneal epithelium, and they are, no doubt, rudimentary 
tubules. 

Firbringer in his paper gives no evidence of the origin of 
these cells from the peritoneal epithelium, except a drawing of 
a stage in which the blastema is connected with the peritoneal 
epithelium.! I have also seen this stage, as mentioned above, 


1 Gétte also, in his latest writings on the subject, agrees with Fiirbringer 
as to the origin of the cells which give rise to the mesonephros. But 1 may 
draw attention to the fact that Gotte has held three views on this point, 
the last of which did not appear (see Firbringer, loc. cit.) till 1875, we. 
afier the publication of Balfour and Semper’s works on ‘ Elasmobranchi.’ 


WOLFFIAN DUCT AND BODY IN THE CHICK. 31 


in my sections of the Frog, but have completely failed to 
find the earlier stages of this ingrowth. One would expect to 
see it preceded by a thickening of the very flat cells lining the 
body cavity at this point; one would hardly expect the flat 
cells so specialised to form the lining of the body cavity of 
the young larva suddenly, and without showing any change to 
begin to grow inward. Further, if the cell cords described by 
Fiirbringer in the Salamander are really only rudimentary struc- 
tures belonging to the anterior part of the mesonephros, as is 
certainly the case in the Frog; and if the process which Fiir- 
bringer describes for the posterior part of the mesonephros of 
the Salamander takes place for all fully-developed parts of the 
mesonephros, as is the case in the Frog, then part of the diffi- 
culty caused by the peculiar secondary development of the 
peritoneal funnels disappears. In other words, I believe Fiir- 
' bringer has made a mistake, precisely similar to that which was 
made about the development of the Avian Wolffian body. He 
has seen in the anterior part of a young larva the cell cords 
mentioned above; which were present at a time when there was 
no trace of the posterior part of the mesonephros. He has also 
seen in the hinder part of older larve the blastema of cells 
separate from the peritoneal epithelium from which the Wolffian 
tubules arise. Finally, he has connected these two conditions, 
which are, as I believe, found in different regions of the trunk, 
and has concluded that the cell strings of the anterior part have 
separated from the peritoneal epithelium and given rise to the 
-cell masses of the posterior part which really develop indepen- 
dently of the peritoneal epithelium, and eventually give rise to 
the Wolffian tubules. 

My observations on Teleostei lead me, for similar reasons, to 
assert an origin, 2” sit#, of a continuous blastema, which later, 
breaking up, will give rise to the Wolffian tubules. 

On the other hand, the older observers, including Vogt and 
Rosenberg for Teleostei, Rathke, Johan. Miiller, Reichert, Vogt, 
for Amphibia,” are quoted by Fiirbringer as asserting an origin 
of the tubules as a series of excavations in a blastema of cells 
lying just internal to the segmental (Wolffian) duct. And it 
seems to me that the older observers were,® as in their state- 
ments concerning the development of the mesonephros in the 
chick, not far from the truth. In the Sturgeon my observations 
point to a similar conclusion; in the just-hatched young a few 
mesoblast cells are seen lying internal to the segmental duct. 
These, at a later stage, are replaced by a more compact mass of 

1 ¢ Firbringer,’ loc. cit., p. 46. 


2 Thid., loc. eit., p. 12. 
3 Self, ‘ Quart. Journ. Mic. Sci.,’ April, 1880. 


32 ADAM SEDGWICK. 


cells, occupying the position of which, in a still older animal, 
Wolffian tubules are seen.1 

The point I wish to insist upon is that sufficient proof of an 
actual ingrowth of cell from the peritoneal epithelium has not 
been given; but that it is much more probable that the kidney 
blastema arose 2m s?¢#, in some cases perhaps in continuity with 
the peritoneal lining, and in other cases independently of it, 
but soon becoming united with it to form the nephrostomata. 

The development of the mesonephros in the Amniota has been 
most fully elucidated in the chick.” 

In a recent paper I have described the development of the 
posterior Wolffian tubules from a continuous blastema of cells 
derived from the intermediate cell mass; and in the first part 
of this paper that of the anterior tubules from the cell cords 
left connecting the Wolffian duct and intermediate cell mass. 

Further, in the chick there is a kind of intermediate method 
i development of the tubules of the 12th—15th segments (see 
above). 

The question here again recurs which was asked before: Are 
these tubules of the anterior part of the Avian Wolffian body 
really tubules of the Wolffian body, or have they something 
to do with the head-kidney? Fora discussion of this question 
I must refer below to p. 460. 


The Metanephros. 


In a recent paper? I have attempted to show that the me- 
tanephros, which is found only in the Amniota, is developed from 
a blastema of cells which arises continuously with but behind 
the blastema from which the Wolffian tubules develop. 

Although the blastema which will give rise to the greater part 
of the metanephros arises at a comparatively early stage in de- 
velopment, still it is not till a much later stage that it shifts its 
position, and begins to show signs of developing into the Wolffian 
tubules. This late development of the kidney, which in this 
point to a certain extent resembles the Amphibian mesonephros, 
is a very remarkable fact. I shall return to it again. 

I have thus run over very rapidly the most salient features 
in the development of the various parts of the Vertebrate excre- 
tory system, so far as it is at present known to us. I now turn to 


' Balfour has recently described the existence of solid cords of cells, 
connected wfth the peritoneal epithelium, in the anterior part of the meso- 
nephros of the sturgeon (‘Comp. Embryology,’ vol. il, p. 581). The 
origin of these cords is not clear, neither is it certain that they undergo 
full development. 

2 Loc. cit. 

* Loc. ‘cit. 


WOLFFIAN DUCT AND BODY IN THE CHICK. 33 


a consideration of the bearing which these facts have upon any 
hypothesis as to the phylogenetic connection of these various 
organs. 

But, before so doing, it will be well to consider the nature of 
the problem which presents itself. It is universally admitted 
that the Craniata have had a common ancestor. The problem 
to be solved is contained in these questions: What was the 
structure and development of the excretory system of that an- 
cestor? How has it been modified to produce the excretory 
organs which we see in Vertebrates now living? 

I am but too well aware how complicated and difficult the 
problem is, and how insufficient are the data we at present 
possess to enable us to solve it. Of the two sources (geology 
and embryology) from which we can hope to obtain these 
data, paleontology can throw no light whatever upon the 
primitive Vertebrate or its ancestors, for the Vertebrates 
have apparently an antiquity greater than that of the oldest 
fossil-bearing rocks; and even if there are in existence fos- 
siliferous rocks bearing the remains of the ancestor of Verte- 
brates (excluding Amphioxus), we can hardly hope, when 
they are found, to obtain any knowledge of the ontogenetic 
development or structure of soft parts, and the light which 
paleontology throws upon the later history is at present difficult 
to use in settling questions of this kind,’ so that we are thrown 
almost entirely upon embryology for the facts; but the facts 
which embryology at present supplies us with are quite inade- 
quate to enable us, even approximately, to solve the problem. 


1 In making out the phylogeny of organs which have had an early origin, 
it seems to me that geology can help us in this way (amongst others). 
Those forms which are found in the oldest rocks, and which have existed 
as small isolated groups, very little changed apparently in structure, to the 
present day, probably retain the same method of development now as 
then. By examining the embryology of such living forms we might 
expect to find the development of certain organs different to that in other 
animals belonging to larger living groups. Turning to the Brachiopoda, a 
group of great antiquity, we find a development of the body cavity which 
is shared by but few animals, and which @ priori we regard as the most 
primitive method of development of that organ known. Now, of the 
animals which resemble the Brachiopoda in this respect, Balanoglossus, 
Amphioxus, and Sagitta are soft bodied, and so not found as fossils; but 
their very isolation at the present day, with regard to their relations to 
other groups, suggests that they are survivals of some larger groups, the 
other members of which have undergone so much evolution that their 
relationship is unrecognisable. The other group, Echinodermata, which 
presents this method of development, is found at its greatest development 
in Paleozoic rocks, and has not undergone any very marked changes since 
that time. It seems to me that, by following this line, some very important 
help might be obtained in helping us to decide questions of organ 
phylogeny. 


34 ADAM SEDGWICK. 


But still, such as they are, it seems worth while to put them 
together, and to discuss the conclusions to which they seem to 
oint. 
: Mr. Balfour! has compared the embryonic record to an 
ancient manuscript in which many leaves are missing, many 
moved out of their proper order, and many spurious ones inter- 
polated by later hands. It is the duty of an embryologist to 
try to reconstruct the manuscript and see exactly what it contained 
when it was first written. In doing this he is aided by the fact that 
he has access to many copies of the manuscript, which have 
each been used and altered by very different people. He is thus 
able, by comparing the different copies, and by studying the 
characters, &c., of the people by whom they have been possessed, 
to arrive at a more correct idea as to what the original was like 
than if he had only one copy. 

In studying the various embryonic records we have we can 
pick out certain features common to all, and which may be 
assumed to have had their counterpart in the phylogenetic 
history. But the majority of features have been so altered that 
it is only possible to arrive at anything like a conclusion by 
taking into account the complicated conditions in which the 
animals have lived. 


Discussion of the preceding Facts. 


While the pronephros is characterised by a very similar struc- 
ture and development in all the animals in which it occurs, the 
mesonephros, though possessing in all animals a fairly similar 
adult structure, presents most remarkable differences in develop- 
ment in the different groups. While the mesonephros is uni- 
versally (few Teleostei excepted) present, the pronephros is only 
present in certain forms. Considering first the Ichthyopsida, it 
is at once seen that the presence or absence of a pronephros is 
correlated with another peculiarity. When the pronephros is 
present the egg contains a relatively small amount of food yolk, 
and the young undergo a considerable part of their development 
after leaving the egg; while, when the pronephros is absent, the 
egg contains a very bulky food yolk, and the young undergo 
far the greater part of their development within the egg (Hlas- 
mobranchii). 

Further, again considering the Ichthyopsida, we find that one 
method of development of the mesonephros is found in those 
animals with a pronephros, while the other method is found 
in those animals without a pronephros. Of the two methods of 
development of the mesonephros, while one (that found in 


1 *Comp. Embryology.’ 


a 


WOLFFIAN DUCT AND BODY IN THE CH8HICK, 35 


Elasmobranchii) may be considered as 2m some respects primitive, 
the other must be regarded as very much modified. 

Whatever may have been the phylogenetic origin of the 
Wolffian tubules, the ontogenetic origin, as seen in Amphibia, 
Teleostei, Ganoids, Marsipobranchu, cannot possibly be re- 
garded as in any way approaching the former. We cannot 
suppose that a definite serial organ like the mesonephros de- 
veloped in phylogeny as a series of independent cavities in a 
mass of mesoblastic cells. At any rate, I think I am justified, 
in the present state of our knowledge, in making this statement. 
It is completely opposed to our ideas, and can only be accepted 
when all other hypothesis as to the origin of the mesonephros in 
phylogeny, based on the facts of embryology, have been shown 
to be untenable. 

The tubules of the mesonephros in Elasmobranchii, however, 
in which group they arise from parts of an organ previously 
developed, present a method of development which is not at all 
at variance with our @ priori views as to their phylogenetic 
origin. From considerations of this kind it seems to me a fair 
assumption that the development of the tubules in Elasmo- 
branchs from parts of the body cavity more nearly resembles 
the method by which the organ arose in phylogeny than does 
that of the Wolffian tubules of the remaining Ichthyopsida. 

In Elasmobranchs the Wolffian tubules have a segmental 
arrangement; one is found in each segment. In all probability 
this also is a primitive condition. 

The arrangement of the tubules in the other vertebrata, 
although it does not actually afford support to this view, still it 
does not disprove it. It is a well-known fact that the segmental 
tubes have very rarely a segmental arrangement in the adult or 
even in the embryo. But in this connection it must be remembered 
that the tendency of development always seems to be to render 
that part of the mesonephros, which is going to function in the 
adult as an excretory organ, more compact, z.e. to bring its 
constituent parts closer together. I need only refer to the kid- 
neys of the Urodele Amphibia. Here the posterior part of the 
mesonephros, which is going to function in the adult as kidney, 
becomes distinguished by its size and the course of its ducts 
from the anterior part, and in the female by its size only from 
the anterior part. And Firbringer has shown, in Sa/amandra 
maculata, that in correspondence with the increasing size of the 
posterior region there is found an increased number of primary 
tubules in a segment, as well as of dorsal secondary tubules.! 


1 Spengel however asserts, that in the female of those Amphibia he has 
investigated, the kidney (mesonephros) contains an uniform number of 


36 ADAM SEDGWICK. 


Spengel has also shown that even in different species of one 
genus the number of primary tubules in a segment differs, e.g. 
in Spelerpes variegatus there 1s one primary tubule in a segment, 
in Spelerpes fuscus there are two. 

Further, Furbringer states that in the species investigated by 
him the number of primary tubules in a segment increases with 
the age of the animal. 

“Die Anlagen sind in ihren fritheren Entwickelungsstadien 
leicht zu scheiden ; spiter hingegen lagern sie sich so innig an 
einander, dass eine Abgrenzung unmoglich wird.’”+ 

Finally, there seems to be a distinct relation between the 
closeness of aggregation of the tubules with regard to the body 
segments and the number of segments found between the 
mouth and the anus. 

In the Anourous Amphibia, where there are very few segments 
in the adult in this region, we find a very compact and complex 
kidney. 

In the Urodeles, in which the number of segments is greater, 
the kidney occupies a greater number of segments, and is not 
nearly so compact, while in Ceecilia, in which the anus is almost 
terminal, very few segments being placed behind (tail undiffer- 
entiated), we find that the kidney is segmental, 7. e. one primary 
tubule is found for each segment, and it occupies in the adult 
as many as sixty segments.” 

Turning to the Amniota, we find that in Lacertilia® the 
mesonephros has at first a segmental arrangement, one primary 
tubule for each segment, and although it has not been shown that 
the fully developed mesonephros of lizards has lost this feature, 
still there can be little doubt, considering its resemblance to that 
of Aves, that it has; while in the case of the chick* the 
number of primary tubules in a segment increases with the age 
of the embryo. 

These three facts, viz.—(1) The variability of the number of 
primary tubules in a segment in closely allied forms, (2) the 
increased ® number in a segment as development proceeds, (3) the 
relation between the compactness of the kidney and the number 
of segments over which it extends, all point in the same direc- 
tion. They seem to indicate that the tubules of the Wolffian 


seemental tubules in each segment over its whole area; while in the male, 
he finds that they increase in number behind. 

1 Loe. cit., p. 19. 

2 Spengel. 

3 Braun. 

4 Self, ‘ Quart. Journ. Mier. Sci.,’ April, 1880. 

5 There is no evidence that this is effected by intercalation in the chick 


at any rate. 


WOLFFIAN DUCT AND BODY IN THE CHICK, 37 


body are capable of shifting their position according to the 
wants of the particular species. 

We know very well other organs can do this, and I need only 
mention the anus placed so near the head in frogs, and so far 
off in Cecilia, and it seems only probable that an important 
gland like the kidney should be capable of acquiring a position 
and arrangement of its constituent parts different from the posi- 
tion of their development, if it is advantageous for the per- 
formance of the function of the organ. 

The evidence which at the first look appeared so strong 
against the primitiveness of the Klasmobranch arrangement of 
one primary tubule to each segment proves on examination to 
lose a great part of its force. 

I now come to a difficulty which apparently at present presents 
an insuperable obstacle to a successful solution of the question 
under consideration, viz. What was the structure and deve- 
lopment of the excretory system of the ancestral Vertebrate? 

Assuming that the development of the Elasmobranch mesone- 
phros presents primitive features in the two details already con- 
sidered, its development in a third particular can by no means 
be assumed to be primitive. The fact that the segmental 
duct develops independently of the tubules cannot, in the 
present state of our knowledge, be regarded as primitive. 
Objections of precisely the same kind as those used in arguing 
against the development of the tubules in Amphibia, &c., being 
primitive present themselves here. 

Any phlyogenetic hypothesis which presents difficulties from 
a physiologica] standpoint must be regarded as very provisional 
indeed. ‘The physiological difficulty present in the conception 
that in the evolution the mesonephros has arisen by the fusion of 
two distinct parts, viz. the duct and tubule, is so great that 
until facts are brought forward to show a different origin we 
must consent to admit our total ignorance on this point. I 
think that the observations recorded in the first part of this 
paper on the development of the Avian Wolffian duct and 
anterior tubules are of great interest in this relation. Here 
we have the Wolffian duct and tubules developing in continuity 
in the anterior part of the excretory system, which has been 
always admitted to present the most primitive development. 
But this point I must again keep for later consideration. 

So far, then, the following conclusions have been reached—the 
development of the mesonephros of Hlasmobranchii is in part 
primitive (tubules), and in part very much modified, while the 
development of the mesonephros of Amphibia, Teleostei, &c., 
is in all respects modified. 

Turning to the development of the segmental duct, we find 


38 ADAM SEDGWICK. 


ourselves obliged, for precisely similar reasons to those already 
given in the case of the mesonephros, to suppose that that 
ontogeny is in this respect more primitive in which the duct 
arises aS a continuous groove constricted off from the body 
cavity than that in which it arises as a solid knob (modified 
groove) for only a very small part of its course, and undergoing 
the major part of its early growth quite independently of sur- 
rounding structure. 

In Elasmobranchii that part which develops as a groove 
persists as a groove throughout life (abdominal opening of 
Millerian duct). 

In Amphibia, &c., that part which develops as a groove 
becomes constricted off first in the middle, and then backwards 
and forwards, but in front it is constricted in a manner, 
according to Firbringer not understood, so as to leave the 
variable numbers of openings of the pronephros. 

However this may be, apparently the openings of the prone- 
phros develop as unclosed portions of the anterior end of the 
groove from which the duct arose, and they open into a space 
placed at the root of the mesentery close to the notochord and 
close to the point where in a previous stage the body cavity 
communicated with the muscle plates. 

In the Amphibian, and apparently in the Teleostean, there is 
no marked structure corresponding to the intermediate cell mass 
of Elasmobranchii. The muscle-plate cavity is, after its separa- 
tion from the general body cavity, only separated from the latter ~ 
by a double layer of cells, forming its ventral wall and the wall 
of the body cavity; 2. e. there is no portion of the body cavity 
at first continuous, but subsequently divided up by the coming 
together of its walls into a series of canals connecting the general 
body cavity with the muscle plates. 

Now the glomerulus of the pronephros develops in a part of 
the body cavity anatomically corresponding to the intermediate 
cell mass of Elasmobranchii, only in Amphibia it does not, in 
this region, become divided up into chambers corresponding to 
the segments. 

With this part of the body cavity, from the somatic walls of 
which the original groove arose, the openings of the head-kidney 
communicate. The number of these openings corresponds with 
the number of segments occupied by the pronephros in all those 
animals in which they exceed one, except Myxine; but the 
development of the pronephros in Myxine is not at all known, 
and its adult structure is, on the whole, obscure. 

Turning again to Hlasmobranchs, we find that the anterior 
knob of the segmental duct arises from the intermediate cell 
mass, 7. ¢. from a part of the body cavity corresponding serially 


WOLFFIAN DUCT AND BODY IN THE CHICK, 39 


with that with which in the succeeding segments it later unites 
when the young segmental tubes acquire a communication with 
the segmental duct. 

In Amphibia the segmental duct, when larval life is tolerably 
advanced, opens into a Wolffian tubule, which arises from a mass 
of cells, the origin of which is obscure, but which apparently 
does not appear till after the larva has left the egg. Now the 
Wolffian tubule of an Amphibian is homologous with that of an 
Elasmobranch ; it is similarly constructed, and opens into the 
body cavity at a corresponding point. Hence we are driven to 
the conclusion that the cells from which the Wolffian tubule in 
an Amphibian arise are homologous with the intermediate cell 
mass of an Elasmobranch. 

But in Amphibia these cells are not developed where, if 
Elasmobranch development is primitive, they should be; and 
appear later in a way which gives no clue to their relationship 
to the intermediate cell mass in Elasmobranchii. 

What is the meaning of this extraordinary method of deve- 
lopment ? 

In Elasmobranchs the development of the segmental duct is 
modified, while the development of the mesonephros is primitive 
in its segmental arrangement and origin as a specialised part of 
an organ present at an earlier stage. 

In Amphibia the development of the segmental duct is more 
primitive, but that of the mesonephros very modified, and this 
very latter fact always goes hand in hand with the presence of a 
pronephros. Turning to the pronephros, it is found to develop 
im continuity with the segmental duct. It is found to possess, 
with regard to its openings into the body cavity, a segmented 
structure. It is also found to possess a structure, the glome- 
rulus, resembling extraordinarily closely the glomerulus of an 
ordinary Malpighian body of the mesonephros. This glome- 
rulus lies in a special part of the body cavity, just as a glome- 
rulus of a Malpighian body in the mesonephros of an 
Elasmobranch lies in what from its origin may be called a 
specialised part of the body cavity; and both these specialised 
sections in their anatomical position precisely correspond (see 
above, p. 38). 

With all these similarities can the inference be avoided that 
the head-kidney is descended from the same primitive excretory 
system as the mesonephros, which has appeared early in develop- 
ment to supply the larva with an excretory organ, and has been 
able to retain a more primitive development ? The larva, having 
this, has not wanted the hinder part, and in consequence, having 
all its energy occupied while within the egg in developing those 
organs which it will reall require as a larva, it leaves over the 


40 ADAM SEDGWICK. 


development of the organs not so required until after it is 
hatched ; and in order that it may not be burdened by useless 
organs, the cells from which the tubules after appear and which 
should appear, if keeping the phylogenetic order, quite early in 
embryonic life, in a way already indicated, are reduced so as 
hitherto to have escaped observation. 

It is perfectly true that the pronephros does present peculia- 
rities of structure not presented by the mesonephros, such as the 
unsegmented nature of the glomerulus, and in the fact that the 
tube connecting the cavity im which the glomerulus lies with 
the segmental duct not being coiled. But in the fundamental 
structure, i.e. in the possession of a glomerulus placed close to 
the main vascular channel (aorta), in the segmental arrangement 
of the openings of the segmental duct into the cavity (anatomi- 
cally corresponding in both cases) containing the glomerulus, 
in the cavity containing the glomerulus being a specialised part 
of the body cavity ; in all these points the pronephros and meso- 
nephros resemble each other. 

Assuming for the moment the truth of this suggestion, we 
find the pronephros to present that method of development 
which & priori we are bound to assume would be if it were not 
for disturbing causes, the development of the mesonephros, 
because it represents the most probable method by which the 
mesonephros and its duct can have arisen in phylogeny. 

The question now arises, What are the disturbing causes 
which in Amphibia have so changed the phylogenetic develop- 
ment? The answer has already been given, but I will repeat it 
here. It has been brought about by the action of natural selec- 
tion on the innumerable larve produced, so that only those ani- 
mals reached the adult state which in their prelarval and larval 
development conformed to the type of development we have 
before us. 

Admitting the possibility of both prelarval as well as larval 
development varying at any particular stage, the tendency has 
been to produce a dissimilarity in the early structure of the ex- 
cretory organs of Elasmobranchii and Amphibia greater than 
that which exists in the adult state, a result entirely in opposition 
to what we should expect from the application of that principle 
which has been laid down as regulating embryonic development, 
viz. that embryos of different animals, starting as fairly similar, 
become more and more dissimilar as their development proceeds. 

To get any actual proof from embryonic development in favour 
of the above hypothesis must, from the nature of the case, be 
very difficult. For the very reason of the existence of the pro- 
nephros as an anterior part of the excretory system well marked 
off from the posterior makes it improbable that anything more 


a 


— ea 


a 


WOLFFIAN DUCT AND BODY IN THE CHICK. Al 


than a trace of the hinder part should appear simultaneously in 
embryonic development.with the anterior part. If the rest of 
the mesonephros developed continuously with the duct and 
simultaneously with the pronephros, then, on the above hypo- 
thesis, we should not be able to distinguish a pronephros from 
the hinder part ; and it is opposed to all our ideas of economy to 
suppose that a rudiment of the mesonephros should appear at 
what phylogenetically would be the proper time, remaining over 
as a rudiment in the larva, z.¢. as a useless organ forming 
merely a burden until it was wanted. 

It seems to me that we can only expect, at the very utmost, 
to find a very small trace of the mesonephros in embryonic de- 
velopment at what phylogenetically we should consider, on the 
above hypothesis, to be the proper moment relative to the 
pronephros. 

I have been examining the development of the segmental 
duct in an Amphibian, the frog, to see if at the time of closure 
of the groove of the segmental duct any trace of a discontinuous 
closure such as we find in the head-kidney existed. If the 
pronephros is merely the anterior part of a segmental organ 
of which the mesonephros is the posterior part, and if 
phylogeny is in any way repeated in the development of the 
pronephros, we should expect to find that the discontinuous (seg- 
mented, see above) closure of the pronephros would be repeated 
behind, showing some traces at least of the openings of the 
segmental duct and of the specialised part of the body cavity 
which later forms the Wolffian tubule and contains the glome- 
rulus. So far it cannot be said that my search has been from 
my point of view successful. To get any evidence of what I was 
searching for requires a very complete series of sections in a state 
of preservation favorable for observation. The difficulties pre- 
sented by the embryonic Amphibia in their early stages to sucha 
successful result are very great. In the first place they are 
very brittle, and comparatively very few of the sections, even if 
thick, can be mounted uninjured. Of these, very few, indeed, 
can be obtained perfect, and those so obtained are apparently 
more difficult to see anything in than the thick ones. The cells 
are full of yolk granules which seem to escape and obliterate the 
outlines of the cells from the sight. 

While my results have not been such as to unable me to speak 
with any confidence either one way or the other, yet on the 
whole they have convinced me that a re-examination with a new 
method of the development of the segmental duct in Amphibia, 
&c., would repay the trouble. 

In the chick, on the other hand, the anterior part of the seg- 
mental duct, for the space of five segments, develops exactly in 


42 ADAM SEDGWICK. 


the manner of the segmental duct . and head-kidney of the 
Ichthyopsida. Are the cell cords connecting the duct and peri- 
toneal epithelium in these segments rudimentary Wolffian 
tubules, or are they rudiments of a head-kidney? In the 
absence of a continuous glomerulus opposite them they differ 
from the openings of the pronephros. In their development 
_ they resemble the latter. If they are Wolffian tubules they 
develop quite differently from all other Wolffian tubules. If 
they are rudimentary pronephric funnels, then the chick pos- 
sesses a rudiment of a pronephros which resembles exactly 
the hinder developing Wolffian tubules. 

It seems to me that these structures, under the light of the 
above hypothesis, present no difficulty, and I cannot help thinking 
that the discovery of their method of development is striking 
evidence in its favour. They belong, on that hypothesis, to the 
anterior part of the excretory organ, which has retained the 
primitive method of development originally characterising the 
whole organ. They, in some Avian ancestor, have constituted 
the first developed part of the excretory system, which has been 
utilised by the larva as its excretory organ. Supposing that 
Avian ancestor existed now, we should find that its larva possessed 
an organ which we should call pronephros, having a structure 
less modified probably from the hinder part of the excretory 
system than in the case of the Ichthyopsida, z.e. an organ the 
serial homology of which, with the mesonephros, would no more 
be disputed than is that of the metanephros with the meso- 
nephros. 

It may be objected to this view of the anterior part of the 
Avian excretory system, that it differs in certain marked features 
from the pronephros of other forms. Of these differences the 
most important is, perhaps, the fact that there is always found 
an interval unoccupied by segmental tubes between it and the 
mesonephros. But in Amphibia Salamandra Fiirbringer! dis- 
tinctly states that rudiments, as masses of cells, occupying the 
same relative position to the segmental duct as do segmental 
tubes, are found intervening between the two. If these rudi- 
mentary tubules underwent full development there would be no 
such gap as that we now find between the pro- and mesonephros 
of Amphibia. 

But this difficulty is merely part of another difficulty which it 
seems to me must exist whatever view be taken of the nature of 
the pronephros, namely, why does this organ, so well developed 
in the larva and apparently perfectly well performing the func- 
tions of an excretory organ, atrophy in the adult? And this 
difficulty only seems capable of the unsatisfactory explanation, 

1 Loc. cit. 


WOLFFIAN DUCT AND BODY IN THE CHICK. 43 


that though perfectly well suiting the requirements of the larva, 
its position is unsuitable for the satisfactory performance of 
its functions in the adult. Balfour has suggested! that the 
atrophy of the pronephros is due to its position in that part of 
the body cavity which eventually becomes the pericardium ; 
and has pointed out, as a confirmation of this view, that it 
only persists in the adult of those animals in which it is com- 
pletely shut off from the body cavity, e.g. Teleostei. 

(The enormous size which the pronephros attains in adult 
Teleostei is peculiar, but, coupled with the remarkably feebly 
_ developed mesonephros in the adult, is not astonishing. The 
pronephros seems capable of carrying on all the excretory work 
in some adult Teleostei, in which the mesonephros is not present. 
The absence of the mesonephros in these cases is probably purely 
secondary, and, no doubt, traces of it would be found if a close 
examination were made. The survival of a larval character into 
the adult state is paralleled by the Axolotl’s gills.) 

A second feature of difference between this anterior part of 
the Avian excretory system and the Amphibian pronephros, is 
the absence in the former of a continuous glomerulus. This 
may be abortion from disuse, and does not really present a 
serious difficulty. 

A third feature of difference is that the Avian pronephros 
extends over a much greater area than that of the Ichthyopsida, 
but when I draw attention to the fact that this difference is 
found amongst the various members of the Ichthyopsida them- 
selves, I think it can hardly be looked upon as a difficulty. In 
Teleostei the head-kidney is distinguished by one peritoneal 
opening and a correspondingly short glomerulus. From this we 
have ail stages to the five peritoneal openings of Petromyzon. 

Finally, even if the Avian pronephros did differ in certain 
features from the Ichthyopsidan pronephrus, this can hardly be 
regarded as a serious difficulty. 

The pronephros of Teleostei with its Malpighian capsule 
containing the isolated glomerulus, and with its one peritoneal 
opening, surely differs considerably from the pronephros of the 
frog with its three peritoneal openings and its glomerulus lying 
free in the body cavity. 

Again, without laying too much stress upon it, I point to the 
pronephros of Myxine, which differs still more remarkably from 
that of other types. 

The difficulty presented by the Elasmobranchii, in which the 
tubules, though retaining certain primitive features of develop- 
ment, do not develop in continuity with the duct, is very great, 


© “Comp. Embryology,’ vol. ii. 


4A, ADAM SEDGWICK,. 


and in the present state of our knowledge no satisfactory ex- 
planation, founded on facts of development, can be given of it. 
I will suggest a possible, but entirely rough and hypothetical, 
solution on the lines so far followed. 

Before the Elasmobranchii produced eggs with the large food 
yolk they at present possess, they may have undergone a large 
part of their development in the surrounding medium as free 
larve. These larvee must have left the egg at a time when 
the cavities of the muscle plates were still open to the body 
cavity, and when the segmental duct had only just commenced 
to be formed in front, and before the development of the vascular 
system, and therefore before the glomerulus, the functions of 
which were probably carried on by the walls of the body cavity. 
The segmental duct was quickly developed from a groove into a 
duct, the larve thus precociously developing a recently acquired 
adult structure. With this constitution the larva of the ances- 
tral Elasmobranch quickly developed the rest of its excretory 
system. In consequence of the larva having been hatched at a 
very primitive stage, before the muscle plates were separated 
from the body cavity, certain primitive characters in the develop- 
ment of the segmental tubes were retained. These characters 
have been more or less transmitted to the present day, this 
having been rendered possible by the acquisition of food yolk 
and abolition of the larval state. 

However this may be, and it is useless now to make hypo- 
theses of this kind, we can only wait till a more close study of 
Elasmobranch development has been made to see if any traces 
can be found of the disturbing cause which has produced the 
modification in the development of the excretory system assumed 
on the above hypothesis, and very possibly in the search along 
the lines which this hypothesis indicates quite a different view as 
to the phylogeny of the vertebrate excretory system may pre- 
sent itself. 

Before concluding I will briefly state what I think to have 
been the structure of the primitive excretory system in the 
ancestral Vertebrate. 

There was a duct occupying the position of the segmental 
duct, z.e. at the dorsal outer angle of the body cavity, at the 
point where the latter becomes separated from the cavities of 
the muscle plates. This duct opened in each segment into the 
dorsal part of the body cavity. On the inner wall of the latter 
projected on each side a vascular ridge formed by the aorta. 
Behind, the segmental duct opened into the cloaca. 

As differentiation proceeded the vascular aortic ridge became 
more especially developed opposite each opening of the segmental 
duct, and parts of each of these enlargements became succes- 


WOLFFIAN DUCT AND BODY IN THE CHICK, 45 


sively enclosed in a special part of the body cavity, giving rise 
to the commencement of the secondary glomeruli. With this 
division of the glomerulus segmentally, and of each segment of 
it into further secondary glomeruli, each lying in a specialised 
part of the body cavity, the openings of the segmental duct 
began to fold and divide, incompletely at first, into special open- 
ings, one for each secondary glomerulus. Finally, this division 
was completed, and the segmental duct communicated by a 
number of openings in each segment with specialised parts of 
the body cavity containing a portion of the original aortic ridge. 
_ The specialised parts containing these glomeruli being still open 
to the body cavity, and the glomeruli being still all distinctly 
attached by a common stalk to the walls of the body cavity, and 
the intermediate parts of the original continuous ridge baving 
completely vanished, now the capsules enclosing the glomeruli 
became more and more completely marked off from the body 
cavity. The openings putting them in communication with the 
segmental duct elongated into tubules which became coiled, and 
the glomeruli themselves gained a greater independence of each 
other by a development of intermediate tissue. 

A trace of the original state of things has descended to the 
present time in the pronephros, with its continuous glomerulus 
opposite the opening of the segmental duct, and placed in a 
specialised part of the body cavity. Differences in structure 
from the supposed primitive state of things have of course arisen, 
in consequence of the specialisation of the pronephros as the 
larval excretory organ. 

In the same way a trace of the division of the primary 
glomeruli into primary, secondary, &c., glomeruli, is left 
in the curious development of the external glomeruli of the 
anterior part of the Avian mesonephros. Only in this case no 
cause can apparently be given for the retention of this primitive 
feature of development. 

An examination of an early stage in the development of the 
Avian Wolffian tubules, when the primary and secondary 
tubules are both fairly well established, but not very compli- 
cated in structure, points very distinctly to the fact that the 
glomeruli of the two tubules are parts of one primitive glome- 
rulus. ‘They appear to be continuous, and while one looks 
ventrally, 2. ¢. the so-called primary glomerulus, the other looks 
dorsally. A glance at the accompanying woodcut will make 
this clear. 

If this drawing of a section through the Wolffian body of a 
chick in a part with primary and secondary tubules, be compared 
with fig. 24, which is from the anterior part of the same chick 
where there are no secondary tubules, it will be seen that the 


46 ADAM SEDGWICK. 


step between them is not great.! It is merely necessary to 
suppose the division of the glomerulus (in fig. 24) into two 
parts, and a simultaneous development of certain folds from the 


ay) 


Ky Lar root 177 


v 
0 


—SsSoUT TO 


GQ 


Wolffian duct to form the tubules, and the original single 
tubule would have been transformed into a ventral primary 
and a dorsal secondary tubule. 

Further, as I have pointed out in another paper,’ the secon- 
dary tubule always arises in close proximity, apparently from 
a blastema continuous with a part of that from which the 
primary tubule arose. 

A modification of development is to be expected, because in 
those animals in which the mesonephros develops after hatching, 
it clearly comes gradually into use. The whole is not wanted 
at once, but with the increasing size of the larva, more tubules 
are wanted. ‘The first developed (primary) in Salamandra 
acquire a structure with which they can apparently perform 
their function when there is hardly a trace of the secondary 
tubules (Furbringer, loc. cit., fig. 26). 


' It will be observed that in this figure the tubule connecting the 
Wolffian duct and capsule is hardly developed. In all probability, this was 
on the analogy of the pronephros, the primitive state of things, the tubule, 
being a secondary differention of the duct near each glomerulus. 

?* Quart. Joura. Mier. Sci.,’ April, 1880. 


WOLFFIAN DUCT AND BODY IN THE CHICK. 47 


A cause of abbreviation is so clear in this case that I need not 
waste time in stating it. 

But the whole details of the development of the secondary, 
&c., dorsal tubules needs reworking, for, with the exception of 
the observation of Mr. Balfour’s for Elasmobranchs, we have no 
real knowledge of their exact method of development. The result 
of such an investigation cannot but be exceedingly interesting 
from a phylogenetic standpoint. 

I cannot help thinking, as before stated, that the development 
of the external glomeruli in the chick may have some interest in 
this relation. 

The modification of the mesonephros of the Amniota is, on 
the above hypothesis, due to the fact that some Avian ancestor 
possessed a larva in which the anterior part of the excretory 
system was early developed, the development of the hinder part 
being deferred, and consequently modified, just as we see to be 
the case now in the Ichthyopsida. 

The still greater modification und retardation of the develop- 
ment of the metanephros or true kidney of the Amniota, and 
the great size which the Wolffian body reaches in the embryo, 
are striking facts which demand consideration in any discussion 
of the Vertebrate excretory system. 

In my paper on the “ Development of the Kidney” I have stated 
my views on the relation of the Amniote kidney to the mesone- 
phros. But one point in that paper is left untouched. 

Why does the kidney appear so late? and also why does the 
Wolffian body become so large and complex—so much larger 
than the small-sized chicks, in which it is fully developed, can 
need ? 

And, further, why should this organ, apparently so well 
adapted to serve as the excretory organ of the adult chick, 
atrophy P 

It may be said, in answer to the Jatter question, that only 
those tubules of the mesonephros which open into the cloaca 
independently of the Wolffian duct can function in the adult, as 
those which have not so changed their course would interfere 
with the function which the Wolffian duct later acquires—the 
carriage of semen. 

It seems to me that the only answer which can be given to 
the first of these questions is this: 

The kidney is thrown back in development for the same 
reason that the mesonephros of the Amphibia is, viz. because 
the ancestor of the chick underwent part of its development out 
of the egg, at which stage of development the testis, not being 
developed, did not interfere with the excretory functions of the 
Wolffian tubules, or vice versa. The large size of the mesone- 


48 ADAM SEDGWICK. 


phros, then, is to be explained on the supposition that the larva 
of the chick’s ancestor used it for a considerable period of its 
early life as an excretory organ, so that it may be said that the 
pronepliros holds the same general relation to the mesonephros 
in the Ichthyopsida as does the mesonephros to the metane- 
phros in the Amniota. 

I do not mean to affirm that the above explanation of the 
lateness of the development of the metanephros is absolutely 
valid, for I think that a careful consideration of the develop- 
ment of the hind part of the mesonephros in Amphibia and Elas- 
mobranchii might necessitate a slightly different explanation. 

But an explanation of that kind must be sought to explain the 
remarkably late development in the chick of an organ which 
phylogenetically must be assumed to have had an origin simul- 
taneous with that of the mesonephros. 

With regard to the relation which the testes enters into with 
the mesonephros, it is interesting to notice the modified develop- 
ment which always characterises this connection. 

Here it can be definitely affirmed that the lateness and conse- 
quent modification of the process is due to the fact that the 
apparatus has not been required in the larvee of the Ichthyopsida 
and of the Amniote ancestors, and consequently has been put off 
and modified in development. The explanation is exactly similar to 
that given for the modification in development of the Amphibian 
mesonephros, except that here we are supposed to be able to 
assert with greater reason that the putting off and consequent 
modification is due to the fact that the connection between the 
testes and mesonephros was not wanted sooner, and so was not 
developed. 


Summary of the Hypothesis and main Arguments used. 


The whole of the Vertebrate excretory system, including pro- 
nephros, mesonephros, and metanephros, are derived from a 
primitive organ possessed by the ancestral Vertebrate. This 
organ had a segmental character, and consisted of a duct, the 
segmental duct opening in every segment into the body cavity, 
close to a continuous structure, known now as the glomerulus, 
which was placed close to the main vascular channels and acted 
as an excretory organ. 

The anterior end of this organ was used by the larva, and 
developing more or less with regard to other structures at the 
normal time, retained many primitive features of development 
originally characterising the whole organ, and is known to us as 
the pronephros. ‘The posterior part of the organ had its develop- 
ment delayed with regard to other structures, particularly those 
in connection with which it primitively developed ; the develop- 


WOLFFIAN DUCT AND BODY IN THE CHICK. 49 


ment was consequently modified. This part is known to us as 
the mesonephros. eye 

The same hypothesis was applied to account for the retarda- 
tion and modification of the development of the metanephros 
with regard to the mesonephros in the Amniota. 

The main facts in favour of the hypothesis are— 

1. The development of the segmental tubes in Elasmobranchii 
and of the pronephros and segmental duct of the Ichthyopsida as 
parts of the body cavity. 

2. The obvious modification in development of the meso- 
nephros, accompanying also the presence of a pronephros in 
most of the Ichthyopsida. 

3. The resemblance in structure between the pronephros and 
mesonephros, particular stress being laid on the fact that the 
glomerulus in both glands is developed in anatomically corre- 
sponding, i.e. homologous, parts of the body cavity 

I may point out before leaving the subject that other views con- 
cerning thenature of the pronephros have been expressed by Gegen- 
baur, Firbringer,! and Balfour.2 The two former authors look 
upon the pronephros as having an antiquity greater than that of 
Vertebrates, greater even than that of the segmented ancestors 
of Vertebrates. They regard it as being descended from the 
primitive excretory system possessed by the unsegmented 
ancestor, which has been retained in such forms as Turbellaria 
and Rotifera, the segmented posterior part having been added 
when the segmented state was reached. 


Miillerian Duct. 


Balfour’s views as to the phylogeny of the Miillerian duct 
and its homology throughout the Vertebrata are well known. 
He supposes it is one or, in the chick, more of the head-kidney 
_ openings which have become modified for generative purposes. 

I still adhere to the view expressed in the paper on the 
“ Rudimentary Head-Kidney of the Chick” as to the meaning 
of the peculiar structures at the anterior end of the Miillerian 
duct, and I think that there are grounds, which it is not necessary 
to enter into here, for supposing that the abdominal opening or 
openings of the Miillerian duct have been derived from the 
anterior part of the excretory system after its modification to 
form the pronephros. But I quite admit that a fuller know- 
ledge of the early development of the Elasmobranch segmental 
duct may necessitate an alteration in this view. 


1 Loc. cit. 

? Balfour looks upon it as the most primitive part of the excretory 
system which has been retained by the larva, as so many ancestral organs 
are, long after they have been lost by the adult. ‘Comparative Embryo- 
logy,’ vol. ii. 

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DESCRIPTION OF PLATES IV AND V. 


PLATE IV. 


Fie, 1.—Transverse section through the pelvic fin of an embryo of 
Scyllium belonging to stage P,' magnified 50 diameters. dp, basipterygium ; 
br, fin-ray; m. muscle; 47, horny fibres supporting the peripheral part of 
the fin. 

Fic, 2.—Pelvic fin of a very young female embryo of Scyllium stellare, 
magnified 16 diameters. dp, basipterygium; pw, pubic process of pelvic 
girdle (cut across below) ; 2/, iliac process of pelvic girdle; fo, foramen. 

Fie. 3.—Pelvic fin of a young male embryo of Scyllium stellare, magni- 
fied 16 diameters. dp, basipterygium ; mo, process of basipterygium con- 
tinued into clasper ; </, iliac process of pelvic girdle; pw, pubic section of 
pelvic girdle. 

Fie. 4.—Transverse section through the ventral part of the trunk of an 
embryo Scyllium of stage P, in the region of the pectoral fins, to show how 
the fins are attached to the body, magnified 18 diameters. 47, cartilaginous 
fin-ray ; 4p, basipterygium ; m, muscle of fin; mp, muscle-plate. 

Fic. 5.—Transverse section through the ventral part of the trunk of an 
embryo Scyllium of stage P, in the regiou of the pelvic fin, on the same 
scale as fig. 4. dp, basipterygium ; 47, cartilaginous fin-rays; m, muscle of 
the fins; mp, muscle-plate. 


PLATE V. 


Fie. 6.—Pectoral fin of an embryo of Scyllium canicula, of a stage be- 
tween O and P, in longitudinal and horizontal section (the skeleton of the 
fin was still in the condition of embryonic cartilage), magnified 36 dia- 
meters. dp, basipterygium (eventual metapterygium) ; 7, cartilaginous 
fin-rays ; pg, pectoral girdle in transverse section; /o, foramen in pectoral 
girdle; pe, epithelium of peritoneal cavity. 

Fie. 7.—Transverse section through the pectoral fin of a Scyllium 
embryo of stage P, magnified 50 diameters. dp, basipterygium ; dr, carti- 
laginous fin-ray ; m, muscle; 47, horny fibres. 

Fic. 8.—Pectoral fin of an embryo of Scyllium stellare, magnified 16 dia- 
meters. mp, metapterygium (basipterygium of earlier stage); me.p, rudi- 
ment of future pro- and mesopterygium; sc, cut surface of a scapular 
process; ev, coracoid process ; fo, foramen; 4/7, horny fibres. 

Fic. 9.—Skeleton of the pectoral fin and part of pectoral girdle of a 
nearly ripe embryo of Scyllium stellare, magnified 10 diameters. mp, meta- 
pterygium ; mes, mesopterygium ; pp, propterygium ; er, coracoid process. 


' T employ here the same letters to indicate the stages as in my Mono- 
graph on Klasmobranch Fishes. 


A, ; cxf 


PL.IV. 


Hanhart imp 


FINS OF ELASMOBRANCHS 


eS 


TEL 


ea 
3 
te 
My 

a 


Hanhartimp 


On the DeveLopment of the Sxututon of the Patrep 
Fins of HLASMOBRANCHII, considered in relation to 
its bearings on the Nature of the Limss of the 
Vertesrata. By F. M. Batroor, F.R.S., F.Z.S8., 
Fellow of Trinity College, Cambridge. 


(With Plates IV and V.) 


SomE years ago the study of the development of the soft 
parts of the fins in several Elasmobranch types, more espe- 
cially in Torpedo, led me to the conclusion that the verte- 
brate limbs were remnants of two continuous lateral fins.! 
More or less similar views (which I was not at that time 
acquainted with) had been previously held by Maclise, Hum- 
phry, and other anatomists; these views had not, however, 
met with much acceptance, and diverge in very important 
points from those put forward by me. Shortly after the 
appearance of my paper J. Thacher published two interesting 
memoirs comparing the skeletal parts of the paired and un- 
paired fins.’ 

In these memoirs Thacher arrives at conclusions as to the 
nature of tne fins in the main similar to mine, but on en- 
tirely independent grounds. He attempts to show that the 
structure of the skeleton of paired fins is essentially the same 
as that of the unpaired fins, and in this comparison lays 
special stress on the very simple skeleton of the pelvic fin in 
the cartilaginous Ganoids, more especially in Acipenser and 
Polyodon. He points out that the skeleton of the pelvic fin 
of Polyodon consists essentially of a series of nearly isolated 
rays, which have a strikingly similar arrangement to that of 


1 «Monograph on the Development of Elasmobranch Fishes,’ pp. 101, 
102 


2 J. K. Thacher, “ Median and Paired Fins; a Contribution to the 
History of the Vertebrate Limbs,” ‘Trans. of the Connecticut Acad.,’ 
vol. iii, 1877. 

J. K. Thacher, “‘ Ventral Fins of Ganoids,” ‘Trans. of the Connecticut 
Acad.,’ vol. iv, 1877. : 


52 MR. F. M. BALFOUR ON THE SKELETON 


the rays of the skeletons in many unpaired fins. He sums 
up his views in the following way :! 

“As the dorsal and anal fins were specialisations of the 
median folds of Amphioxus, so the paired fins were speciali- 
sations of the two lateral folds which are supplementary to 
the median in completing the circuit of the body. These 
lateral folds, then, are the homologues of Wolffian ridges in 
embryos of higher forms. Here, as in the median fins, 
there were formed chondroid and finally cartilaginous rods. 
These became at least twice segmented. The orad ones, 
with more or less concrescence proximally, were prolonged 
inwards. The cartilages spreading met in the middle line; 
and a later extension of the cartilages dorsad completed the 
limb-girdle. 

‘‘The hmbs of the Protognathostomi consisted of a series 
of parallel articulated cartilaginous rays. They may have 
coalesced somewhat proximally and orad. In the ventral 
pair they had extended themselves mesiad until they had 
nearly or quite met and formed the hip-girdle; they had 
not here extended themselves dorsad. In the pectoral limb 
the same state of things prevailed, but was carried a step 
further, namely, by the dorsal extension of the cartilage 
constituting the scapular portion, thus more nearly forming 
a ring or girdle.” 

The most important point in Thacher’s theories which I 
cannot accept is the derivation of the folds, of which the 
paired fins of the Vertebrata are supposed to be. specialisa- 
tions, from the lateral folds of Amphioxus; and Thacher 
himself recognises that this part of his theory stands on 
quite a different footing to the remainder. 

Not long after the publication of Thacher’s paper, an 
important memoir was published by Mivart in the ‘ Trans- 
actions’ of the Zoological Society.” The object of the 
researches recorded in this paper was, as Mivart explains, to 
test how far the hard parts of the limbs and of the azygos 
fius may have arisen through centripetal chondrifications or 
calcifications, and so be genetically exoskeletal.3 

Mivart’s investigations and the majority of his views were 
independent of Thacher’s memoir ; but he acknowledges that 
he has derived from Thacher the view that pelvic and pec- 

1 Loe. ‘cit.,, ps 298: 


4 St. George Mivart, “On the Fins of Elasmobranchii,” ‘ Zoological 


Trans.,’ vol. x. 

3 Mivart used the term exoskeletal in an unusual and (as it appears to 
me) inconvenient manner. ‘The term is usually applied to dermal skeletal 
structures; but the skeleton of the limbs, with which we are here con- 
cemnced, is undoubtedly not of- this nature. 


Ris. 


OF THE PAIRED FINS OF ELASMOBRANCHS. 53 


toral girdles, as well as the skeleton of the limbs, may have 
arisen independently of the axial skeleton. 

The descriptive part of Mivart’s paper contains an account 
of the structure of a great variety of interesting and unde- 
scribed types of paired and unpaired fins, mainly of Elasmo- 
branchii. The following is the summary given by Mivart of 
the conclusions at which he has arrived :1 

“1. Two continuous lateral longitudinal folds were deve- 
loped similar to dorsal and ventral median longitudinal 
folds. 

«2. Separate narrow solid supports (radials), in longitu- 
dinal series, and with their long axes directed more or less 
outwards at right angles with the long axis of the body, 
were developed in varying extents in all these four longitu- 
dinal folds. 

“<3. The longitudinal folds became interrupted variously, 
but so as to form two prominences on each side, i.e. the 
primitive paired limbs. 

*‘4, Each anterior paired limb increased in size more 
rapidly than the posterior limb. 

“5. The bases of the cartilaginous supports coalesced as 
was needed, according to the respective practical needs of 
the different separate portions of the longitudinal folds, 7.e. 
the respective needs of the several fins. 

*©6. Occasionally the dorsal radials coalesced (as in Nofi- 
danus, &c.) and sought centripetally (Pristis, &c.) adherence 
to the skeletal axis. 

*°7. The radials of the hinder paired limb did so more 
constantly, and ultimately prolonged themselves inwards by 
mesiad growth from their coalesced base, till the piscine 
pelvic structure arose, as e.g. in Squatina. 

“8. The pectoral radials with increasing development 
also coalesced proximally, and thence prolonging themselves 
inwards to seek a point d’appui, shot dorsad and ventrad to 
obtain a firm support and at the same time to avoid the 
visceral cavity. Thus they came to abut dorsally against 
the axial skeleton, and to meet ventrally together in the 
middle line below. 

“9, The lateral fins, as they were applied to support the body 
on the ground, became elongated, segmented, and narrowed, 
so that probably the line of the propterygium, or possibly 
that of the mesopterygium, became the cheiropterygial axis. 

“10. The distal end of the incipient cheiropterygium 
either preserved and enlarged pre-existing cartilages or deve- 
loped fresh ones to serve fresh needs, and so grew into the 

1 Loe. cit., p. 480. 


a4 MR. F. M. BALFOUR ON THE SKELETON 


developed cheiropterygium ; but there is not yet enough evi- 
dence to determine what was the precise course of this trans- 
formation. 


11. The pelvic limb acquired a solid connection with the . 


axial skeleton (a pelvic girdle) through its need of a point 
d’appui as a locomotive organ on land. 

“12. The pelvic limb became also elongated; and when 
its function was quite similar to that of the pectoral limb, 
its structure became also quite similar (e.g. Ichthyosaurus, 
Plesiosaurus, Chelydra, &c.) ; but for the ordinary quadru- 
pedal mode of progression it became segmented and inflected 
in a way generally parallel with, but (from its mode of use) 
in part inversely to, the inflections of the pectoral limb.” 

Gunther! has propounded a theory on the primitive cha- 
racter of the fins, which, on the whole, fits in with the view 
that the paired fins are structures of the same nature as the 
unpaired fins. The interest of Gunther’s views on the nature 
of the skeleton of the fins more especially depends upon the 
fact that he attempts to evolve the fin of Ceratodus from the 
typical Selachian type of pectoral fin. His own statement on 
this subject is as follows :? 

‘©On further inquiry into the more distant relations of the 
Ceratodus-limb we may perhaps be justified in recognizing in 
it a modification of the typical form of the Selachian pectoral 
fin. Leaving aside the usual treble division of the carpal 
cartilage (which, indeed, is sometimes simple), we find that 
this shovel-like carpal forms the base for a great number of 
phalanges, which are arranged in more or less regular trans- 
verse rows (zones) and in longitudinal rows (series). The 
number of phalanges of the zones and series varies according 
to the species and the form of the fin; in Cestracion philippt 
the greater number of phalanges is found in the proximal 
zones and middle series, all the phalanges decreasing in size 
from the base of the fin towards the margins. In a Sela- 
chian with a long, pointed, scythe-shaped, pectoral fin, like 
that of Ceratodus, we may, from analogy, presume that the 
arrangement of the cartilages might be somewhat like that 
shown in the accompanying diagram, which I have divided 
into nine zones and fifteen series. 

“ When we now detach the outermost phalanx from each 
side of the first horizontal zone, and with it the other 
phalanges of the same series, when we allow the remaining 
phalanges of this zone to coalesce into one piece (as, in 
nature, we find coalesced the carpels of Ceratodus and many 


' « Description of Ceratodus,” ‘ Phil. Trans.,’ 1871. 
* Loe. cit., p. 534. 


OF THE PAIRED FINS OF ELASMOBRANCHS. 55 


phalanges in Selachian fins), and when we repeat this same 
process with the following zones and outer series, we arrive 
at an arrangement identical with what we actually find in 
Ceratodus.” 

While the researches of Thacher and Mivart are strongly 
confirmatory of the view at which I had arrived with refer- 
ence to the nature of the paired fins, other hypotheses as to 
the nature of the skeleton of the fins have been enunciated, 
both before and after the publication of my memoir, which 
are either directly or indirectly opposed to my view. 

Huxley, in his memoir on Ceratodus, which throws light 
on so many important morphological problems, has dealt 
with the nature of paired fins. 

He holds, in accordance with a view previously adopted by 
Gegenbaur, that the limb of Ceratodus “ presents us with 
the nearest approximation to the fundamental form of verte- 
brate limb or archipterygium,” and is of opinion that in a 
still more archaic fish than Ceratodus the skeleton of the fin 
“would be made up of homologous segments, which might 
be termed pteromeres, each of which would consist of a meso- 
mere with a preaxial and a postaxial paramere.” He con- 
siders that the pectoral fins of Elasmobranchii, more 
especially the fin of Notidanas, which he holds to be the 
most primitive form of Elasmobranch fin, “ results in the 
simplest possible manner from the shortening of the axis of 
such a fin-skeleton as that of Ceratodus, and the coalescence 
of some of its elements.” Huxley does not enter into the 
question of the origin of the skeleton of the pelvic fin of 
Elasmobranchii. 

It will be seen that Huxley’s idea of the primitive structure 
of the archipterygium is not easily reconcilable with the 
view that the paired fins are parts of a once continuous 
lateral fin, in that the skeleton of such a lateral fin, if it 
has existed, must necessarily have consisted of a series of 
parallel rays. 

Gegenbaur? has done much more than any other living 


1T. H. Huxley, “On Ceratodus Fosteri, with some Observations on the 
Classification of Fishes,’’ ‘ Proc. Zool. Soc.,’ 1876. 

2 C. Gegenbaur, ‘ Untersuchungen z. vergleich. Anat. d. Wirbelthiere ’ 
(Leipzig, 1864-5): erstes Heft, Carpus u. Tarsus; zweites Heft, Brust 
flosse d. Fische. 

“Ueb. d. Skelet d. Gliedmaassen d. Wirbelthiere im Allgemeinen u. d. 
Hintergliedmaassen d. Selachier insbesondere,” ‘ Jenaische Zeitschrift,’ vol. 
v, 1870. 

“Ueb. d. Archipterygium,”’ ‘Jenaische Zeitschrift,’ vol. vii, 1873. | 

“Zur Morphologie d. Gliedmaassen d. Wirbelthiere,’’ ‘ Morphologisches 
Jahrbuch,’ vol. ii, 1876. 


56 MR. F. M. BALFOUR ON THE SKELETON 


anatomist to elucidate the nature of the fins; and his views 
on this subject have undergone considerable changes in the 
course of his investigations. After Giinther had worked out 
the structure of the fin of Ceratodus, Gegenbaur suggested that 
it constituted the most primitive persisting type of fin, and 
has, moreover, formed a theory as to the origin of the fins 
founded on this view, to the effect that the fins, together 
with their respective girdles, are to be derived from visceral 
arches with their rays. 

His views on this subject are ae explained in the 
subjoined passages quoted from the English translation of 
his ‘ Elements of Comparative Anatomy,’ pp. 473 and 477. 

“The skeleton of the free appendage is attached to the extre- 
mity of the girdle. When simplest, this is made up of cartilagi- 
nous rods (rays), which differ in their size, segmentation, and 
relation to one another. One of these rays is larger than 
the rest, and has a number of other rays attached to its sides. 
I have given the name of archipterygium to the ground-form 
of the skeleton which extends from the limb-bearing girdle 
into the free appendage. The primary ray is the stem of 
this archipterygium, the characters of which enable us to 
follow out the lines of development of the skeleton of the 
appendage. Cartilaginous arches beset with the rays form 
the branchial skeleton. The form of skeleton of the append- 
ages may be compared with them; and we are led to the 
conclusion that it is possible that they may have been 
derived from such forms. In the branchial skeleton of the 
Selachii the cartilaginous bars are beset with simple rays. 
In many a median one is developed to a greater size. As 
the surrounding rays become smaller, and approach the 
larger one we get an intermediate step towards that arrange- 
ment in which the larger median ray carries a few smaller 
ones, This differentiation of one ray, which is thereby 
raised to a higher grade, may be connected with the primitive 
form of the appendicular skeleton ; and as we compare the 
girdle with a branchial arch, so we may compare the median 
ray and its secondary investment of rays with the skeleton © 
of the free appendage. 

“ All the varied forms which the skeleton of the free 
appendages exhibits may be derived from a ground-form 
which persists in a few cases only, and which represents the 
first, and consequently the lowest, stage of the skeleton in 
the fin—the archipterygium. This is made up of a stem 
which consists of jointed pieces of cartilage, which is articu- 
lated to the shoulder-girdle and is beset on either side with 
rays which are likewise jointed. In addition to the rays of 


OF THE PAIRED FINS OF ELASMOBRANCHS. - oe 


the stem there are others which are directly attached to the 
hmb- girdle. 

“* Ceratodus has a fin-skeleton of this form ; in it there is 
a stem beset with two rows of rays. But there are no rays 
in the shoulder-girdle. This biserial investment of rays on 
the stem of the fin may also undergo various kinds of modi- 
fications. Among the Dipnoi, Protopterus retains the medial 
row of rays only, which have the form of fine rods of 
cartilage ; in the Selachii, on the other hand, the lateral 
rays are considerably developed. The remains of the medial 
row are ordinarily quite small, but they are always sufliciently 
distinct to justify us in supposing that in higher forms the 
two sets of rays might be better developed. Rays are still 
attached to the stem and are connected with the shoulder- 
girdle by means of larger plates. The joint of the rays are 
sometimes broken up into polygonal plates, which may 
further fuse with one another; concrescence of this kind 
may also affect the pieces which form the base of the fin. 
By regarding the free rays, which are attached to these 
basal pieces, as belonging to these basal portions, we are 
able to divide the entire skeleton of the fin into three 
segments—pro-, meso-, and metapterygium. 

“The metapterygium represents the stem of the archi- 
pterygium and the rays on it. The propterygium and the 
mesopterygium are evidently derived from the rays which 
still remain attached to the shoulder-girdle.” 

Since the publication of the memoirs of Thacher, Mivart, 
and myself, a pupil of Gegenbaur’s, M. v. Davidoff,! has 
made a series of very valuable observations, in part directed 
towards demonstrating the incorrectness of our theoretical 
views, more especially Thacher’s and Mivart’s view of the 
genesis of the skeleton of the limbs. Gegenbaur? has also 
written a short paper in connection with Davidoff’s memoir, 
in support of his own as against our views. 

It would not be possible here to give an adequate account 
of Davidoff’s observations on the skeleton, muscular system, 
and nerves of the pelvic fins. His main argument against 
the view that the paired fins are the remains of a continuous 
lateral fin is based on the fact that a variable but often 
considerable number of the spinal nerves in front of the 
pelvic fin are united by a longitudinal commissure with the 
true plexus of the nerves supplying the fin. From this he 


1 M. v. Davidoff, “ Beitrage z. vergleich. Anat. d. hinteren Gliedmaassen 
d. Fische I.,’’ ‘ Morpho!. Jahrbuch,’ vol. v, 1879. 

2 «Zur Gliedmaassenfrage. An die Untersuchungen von Davidoff’s 
angekniipfte Bemerkungen,” ‘ Morphol. Jahrbuch,’ vol. v, 1879. 


58 MR. F. M. BALFOUR ON THE SKELETON 


concludes that the pelvic fin has shifted its position, and 
that it may once therefore have been situated close behind 
the visceral arches. Granting, however, that Davidoff’s 
deduction from the character of the pelvic plexus is correct, 
there is, so far as I see, no reason in the nature of the 
lateral-fin theory why the pelvic fins should not have shifted ; 
and, on the other hand, the longitudinal cord connecting 
some of the ventral roots in front of the pelvic fin may have 
another explanation. It may, for instance, be a remnant 
of the time when the pelvic fin had a more elongated form 
than at present, and accordingly extended further forwards. 

In any case our knowledge of the nature and origin of 
nervous plexuses is far too imperfect to found upon their 
characters such conclusions as those of Davidoff. 

Gegenbaur, in his paper above quoted, further urges 
against Thacher’s and Mivart’s views the fact that there is 
no proof that the fin of Polyodon is a primitive type; and 
also suggests that the epithelial line which I have found 
connecting the embryouic pelvic and pectoral fins in Torpedo 
may be a rudiment indicating a migration backwards of the 
pelvic fin. 

With reference to the development of the pectoral fin in 
the Teleostei, there are some observations of ’Swirski,! 
which unfortunately do not throw very much light upon the 
nature of the limb. 

*Swirski finds that in the Pike the skeleton of the limb is 
formed of a plate of cartilage continuous with the pectoral 
girdle, which soon becomes divided into a proximal and a 
distal portion. The former is subsequently segmented into 
five basal rays, and the later into twelve parts, the number 
of which subsequently becomes reduced. 

The observations recorded in the present paper were made 
with the object of determining how far the development of 
the skeleton of the limbs throws light on the points on which 
the anatomists whose opinions have just been quoted are 
at variance. 

They were made, in-the first instance, to complete a 
chapter in my work on comparative embryology; and, 
partly owing to the press of other engagements, but still 
more to the difficulty of procuring material, my observations 
are confined to the two British species of the genus Scyllium, 
viz. Sc. stellare and Sc. canicula ; yet I venture to believe 
that the results at which I have arrived are not wholly 
without interest. 


1G. ’Swirski, ‘Untersuch. tb. d. Entwick. d. Schultergurtels u. d. 
Skelets d. Brusiflosse d. Hechts.,’ Inaug. Diss., Dorpat, 1880. 


OF THE PAIRED FINS OF ELASMOBRANCHS. 59 


Before dealing with the development of the skeleton of 
the fin, it will be convenient to describe with great brevity 
the structure of the pectoral and pelvic fins of the adult. 
The pectoral fins consist of broad plates inserted horizontally 
on the sides of the body; so that in each there may be dis- 
tinguished a dorsal and a ventral surface, and an anterior 
and a posterior border. Their shape may best be gathered 
from the woodcut (fig. 1); and it is to be especially noted 


Fie. 1. 


\hi \ 
At 


\\\ 
\ \ 
\\ NY i 


i 
| 


Hk Aj 
tet 


Pectoral fins and girdle of an adult of Scy//iam canicula 
(natural size, seen from behind and above.) 
co. Coracoid ; sc. scapula; pp. propterygium ; me p. mesopterygium ; 
mp. metapterygium ; fz. part of fin supported by horny fibre. 


Right pelvic fin and part of pelvic girdle of an adult female of Scy/lium 
canicula (natural size). 


il. Iliac process ; pz. pubic process, cut across below ; 4p. basipterygium ; 
af. anterior cartilaginous fin-ray articulated to pelvic girdle ; fx. part of fin 
supported by horny fibres. 


that the narrowest part of the fin is the base, where it is 
attached to the side of the body, ‘The cartilaginous skeleton 


60 MR. F. M. BALFOUR ON THE SKELETON 


only occupies a small zone at the base of the fin, the 
remainder being formed of a fringe supported by radiately 
arranged horny fibres.! 

The true skeleton consists of three basal pieces articulating 
with the pectoral girdle; on the outer side of which there is 
a series of more or less segmented cartilaginous fin-rays. 
Of the basal cartilages one (pp) is anterior, a second (mep) 
is placed in the middle, and a third is posterior (mp): They 
have been named by Gegenbaur the propterygium, the meso- 
pterygium, and the metapterygium ; and these names are now 
generally adopted. 

The metapterygium is by far the most important of the 
three, and in Scyllium canicula supports twelve or thir- 
teen rays.” It forms a large part of the posterior boundary 
of the fin, and bears rays only on its anterior border. 

The mesopterygium supports two or three rays, in the 
basal parts of which the segmentation into distinct rays is 
imperfect ; and the propterygium supports only a single ray. 

The pelvic fins are horizontally placed, hke the pectoral 
fins, but differ from the latter in nearly meeting each other 
along the median ventral line of the body. They also differ 
from the pectoral fins in having a relatively much broader 
base of attachment to the sides of the body. Their carti- 
laginous skeleton (woodcut, fig. 2) consists of a basal bar, 
placed parallel to the base of the fin, and articulated in front 
with the pelvic girdle. 

On its outer border it articulates with a series of carti- 
laginous fin-rays. I shall call the basal bar the basipterygium. 
The rays which it bears are most of them less segmented 
than those of the pectoral fin, being only divided into two; 
and the posterior ray, which is placed in the free posterior 
border of the fin, continues the axis of the basipterygium. 
In the male it is modified in connection with the so-called 
clasper. 

The anterior fin-ray of the pelvic fin, which is broader than 
the other rays, articulates directly with the pelvic girdle, 
instead of with the basipterygium. This ray, in the female 
of Scyllium canicula aud in the male of Scyllium catulus 
(Gegenbaur), is peculiar in the fact that its distal segment 
is longitudinally diyided into two or more pieces, instead of 


1 The horny fibres are mesoblastic products; they are formed, in the first 
instance, as extremely delicate fibrils on the inner side of the membrane 
separating the epiblast from the mesoblast. 

* In one example where the metapterygium had 13 rays the mesoptery- 
gium had only 2 rays. 


_ On ee eee ————E— 


OF THE PAIRED FINS OF ELASMOBRANCHS. 61 


being single as is the case with the remaining rays. It is 
probably equivalent to two of the posterior rays. 

Development of the paired Fins.—The first rudiments of 
the limbs appear in Scyllium, as in other fishes, as slight 
longitudinal ridge-like thickenings of the epiblast, which 
closely resemble the first rudiments of the unpaired fins. 

These ridges are two in number on each side—an anterior 
immediately behind the last visceral fold, and a posterior on 
the level of the cloaca. In most Fishes they are in no way 
connected ; but in some Elasmobranch embryos, more especi- 
ally in that of Torpedo, they are connected together at their 
first development by a line of columnar-epiblast cells. This 
connecting line of columnar epiblast, however, is a very 
transitory structure. The rudimentary fins soon become 
more prominent, consisting of a projecting ridge both of 
epiblast and mesoblast, at the outer edge of which is a fold 
of epiblast only, which soon reaches considerable dimensions. 
At a later stage the mesoblast penetrates into this fold, and 

_the fin becomes a simple ridge of mesoblast covered by 
epiblast. The pectoral fins are at first considerably ahead of 
the pelvic fins in development. 

The direction of the original epithelial line which connected 
the two fins of each side is nearly, though not quite, longi- 
tudinal, sloping somewhat obliquely ventralwards. It thus 
comes about that the attachment of each pair of limbs is 
somewhat on a slant, and that the pelvic pair nearly meet 
each other in the median ventral line shortly behind the 
anus. 

The embryonic muscle-plates, as I have elsewhere shown, 
grow into the bases of the fins; and the cells derived from 
these ingrowths, which are placed on the dorsal and ventral 
surfaces in immediate contact with the epiblast, probably 
give rise to the dorsal and ventral muscular layers of the 
limb, which are shown in section in Plate LV, fig. 1m and in 
Plate V, fig. 7m. 

The cartilaginous skeleton of the limbs is developed in the 
indifferent mesoblast cell between the two layers of muscles, 
Its early development in both the pectoral and the pelvic 
fins is very similar. When first visible it differs histolo- 
-gically from the adjacent mesoblast simply in the fact of its 
cells being more concentrated ; while its boundary is not 
sharply marked. 

At this stage it can only be studied by means of sections. 
It arises simultaneously and continuously with the pectoral 
and pelvic girdles, and consists, in both fins, of a bar 
springing at right angles from the posterior side of the pec- 


62 MR. F. M. BALFOUR ON THE SKELETON 


toral or pelvic girdle, and running parallel to the long axis 
of the body along the base of the fin. The outer side of this 
bar is continued into a thin plate, which extends into the 
fin. 

The structure of the skeleton of the fin slightly after its 
first differentiation will be best understood from Plate IV, 
fig. 1, and Plate V, fig. 7. These figures represent trans- 
verse sections through the pelvic and pectoral fins of the 
same embryo on the same scale. The basal bar is seen at 
bp, and the plate at this stage (which is considerably later 
than the first differentiation) already partially segmented 
into rays at br. Outside the region of the cartilaginous 
plate is seen the fringe with the horny fibres (hf); and dor- 
sally and ventrally to the cartilaginous skeleton are seen the 
already well-differentiated muscles (m). 

The pectoral fin is shown in horizontal section in Plate 
V, fig. 6, at a somewhat earlier stage than that to which the 


transverse sections belong. The pectoral girdle (p.g) is cut. 


transversely, and is seen to be perfectly continuous with the 
basal bar (dp) of the fin. A similar continuity between the 
basal bar of the pelvic fin and the pelvic girdle is shown in 
Plate IV, fig. 2, at a somewhat later stage. The plate con- 
tinuous with the basal bar of the fin is at first, to a consider- 
able extent in the pectoral, and to some extent in the pelvic 
fin, a continuous lamina, which subsequently segments into 
rays. In the parts of the plate which eventually form dis- 
tinct rays, however, almost from the first the cells are more 
concentrated than in those parts which will form the tissue 
between the rays; and I am not inclined to lay any stress 
whatever upon the fact of the cartilaginous fin-rays being 
primitively part of a continuous lamina, but regard it as a 
secondary phenomenon, dependent on the mode of conversion 
of embryonic mesoblast cells into cartilage. In all cases the 
separation into distinct rays is to a large extent completed 
before the tissue of which the plates are formed is sufficiently 
differentiated to be called cartilage by an histologist. 

The general position of the fins in relation to the body, 
and their relative sizes, may be gathered from Plate IV, figs. 
4 and 5, which represent transverse sections of the same 
embryo as that from which the tranverse sections showing 
the fin on the larger scale were taken. 

During the first stage of its development the skeleton of 
both fins may thus be described as consisting of a longi- 
tudinal bar running along the base of the fin, and giving off at 
right angles series of rays which pass into the fin. The 
longitudinal bar may be called the basipterygium ; and it is 


—— 


> 


OF THE PAIRED FINS OF ELASMOBRANCHS., 63 


continuous-in front with the pectoral or pelvic girdle, as the 
case may be. 

The further development of the primitive skeleton is 
different in the case of the two fins. 

The Pelvic Fin.—The changes in the pelvic fin are com- 
paratively slight. Plate IV, fig. 2 is a representation of the 
fin and its skeleton in a female of Scyllium stellare shortly 
after the primitive tissue is couverted into cartilage, but 
while it is still so soft as to require the very greatest care in 
dissection. The fin itself forms a simple projeetion of the 
side of the body. The skeleton consists of a basipterygium 
(dp), continuous in front with the pelvic girdle. To the 
outer side of the basipterygium a series of cartilaginous fin- 
rays are attached—the posterior ray forming a direct pro- 
longation of the basipterygium, while the anterior ray is 
united rather with the pelvic girdle than with the basi- 
pterygium. All the cartilaginous fin-rays except the first are 
completely continuous with the basipterygium, their struc- 
ture in section being hardly different from that shown in 
Plate IV, fig. 1. 

The external form of the fin does not change very greatly 
in the course of the further development; but the hinder 
part of the attached border is, to some extent, separated off 
from the wall of the body, and becomes the posterior border 
of the adult fin. With the exception of a certain amount of 
segmentation in the rays, the character of the skeleton 
remains almost as in the embryo. The changes which take 
place are illustrated by Plate 1V, fig. 3, showing the fin of 
a young male of Scyllium stellare. The basipterygium has 
become somewhat thicker, but is still continuous in front 
with the pelvic girdle, and otherwise retains its earlier 
characters. The cartilaginous fin-rays have now become 
segmented off from it and from the pelvic girdle, the poste- 
rior end of the basiterygial bar being segmented off as the 
terminal ray. 

The anterior ray is directly articulated with the pelvic 
girdle, and the remaining rays continue articulated with the 
basipterygium. Some of the latter are partially segmented. 

As may be gathered by comparing the figure of the fin at 
the stage just described with that of the adult fin (woodcut, 
fig. 2), the remaining changes are very slight. The most 
important is the segmentation of the basipterygial bar from 
the pelvic girdle. 

The pelvic fin thus retains in all essential points its 
primitive structure. 

The Pectoral Fin.—The earliest stage of the pectoral fin 


64 MR. F. M. BALFOUR ON THE SKELETON 


differs, as 1 have shown, from that of the pelvic fin only in 
minor points (Plate V, fig. 6). There is the same longitu- 
dinal or basipterygial bar (dp), to which the fin-rays are 
attached, which is continuous in front with the pectoral 
girdle (pg). The changes which take place in the course 
of the further development, however, are very much more 
considerable in the case of the pectoral that in that of the 
pelvic fin. 

The most important change in the external form of the 
fin is caused by a reduction in the length of its attachments 
to the body. At first (Plate V, fig. 6), the base of the fin 
is as long as the greatest breadth of the fin; but it gradually 
becomes shortened by being constricted off from the body at 
its hinder end. In connection with this process the posterior 
end of the basipterygial bar is gradually rotated outwards, 
its anterior end remaining attached to the pectoral girdle. 
In this way this bar comes to form the posterior border of 
the skeleton of the fin (Plate V, figs. 8 & 9), constituting 
the metapterygium (mp). It becomes eventually segmented 
off from the pectoral girdle, simply articulating with its 
hinder edge. 

The plate of cartilage, which is continued outwards from 
the basipterygium, or, as we may now call it, the meta- 
pterygium, into the fin, is not nearly so completely divided 
up into fin-rays as the homologous part of the pelvic fin ; 
and this is especially the case with the basal part of the 
plate. ‘This basal part becomes, in fact, at first only divided 
into two parts (Plate V, fig. 8)—a small anterior part at the 
front end (me.p), and a larger posterior along the base of the 
metapterygium (mp) ; and these two parts are not completely 
segmented from each other. The anterior part directly 
joins the pectoral girdle at its base, resembling in this respect 
the anterior fin-ray of the pelvic girdle. It constitutes the 
(at this stage undivided) rudiment of the mesopterygium 
and propterygium of Gegenbaur. It bears in my specimen 
of this age four fin-rays at its extremity, the anterior not 
being well marked. The remaining fin-rays are prolonga- 
tions outwards of the edge of the plate continuous with the 
metapterygium. These rays are at the stage figured more 
or less transversely segmented, but at their outer edge they 
are united together by a nearly continuous rim of cartilage. 
The spaces between the fin-rays are relatively considerably 
larger than in the adult. 

The further changes in the cartilages of the pectoral 
limb are, morphologically speaking, not important, and are 
easily understood by reference to Plate V, fig. 9 (representing 


OF THE PAIRED FINS OF ELASMOBRANCHS. 65 


the skeleton of the limb of a nearly ripe embryo). The 
front end of the anterior basal cartilage becomes segmented 
off as a propterygium (pp), bearing a single fin-ray, leaving 
the remainder of the cartilage as a mesopterygium (mes). 
The remainder of the now considerably segmented fin-rays 
are borne by the metapterygium. 

General Conclusions.—From the above observations, con- 
clusions of a positive kind may be drawn as to the primitive 
structure of the skeleton ; and the observations have also, it 


‘appears to me, important bearings on the theories of my 


predecessors in this line of investigation. . 

The most obvious of the positive conclusions is to the 
effect that the embryonic skeleton of the paired fins consists 
of a series of parallel rays similar to those of the unpaired 
fins. These rays support the soft parts of the fins, which 
have the form of a loagitudinal ridge; and they are con- 
tinuous at their base with a longitudinal bar. This bar, 
from its position at the base of the fin, can clearly never 
have been a medium axis with the rays on both sides. It 
becomes the basipterygium in the pelvic fin, which retains 
its embryonic structure much more completely than the 
pectoral fin; and the metapterygium in the pectoral fin. 
The metapterygium of the pectoral fin is thus clearly homo- 
logous with the basipterygium of the pelvic fin, as originally 
supposed by Gegenbaur, and has since been maintained by 
Mivart. The propterygium and mesopterygium are obviously 
relatively unimportant parts of the skeleton as compared 
with the metapterygium. 

My observations on the development of the skeleton of 
fins certainly do not of themselves demonstrate that the 
paired fins are remuauts of a once continuous lateral fin ; 
but they support this view in that they show the primitive 
skeleton of the fins to have exactly the character which 
might have been anticipated if the paired fins had originated 
from a continuous lateral fin. The longitudinal bar of the 
paired fins is believed by both Thacher and Mivart to be 
due to the coalescence of the bases of the primitively 
independent rays of which they believe the fin to have been 
originally composed. This view is probable enough in 
itself, and is rendered more so by the fact, pointed out by 
Mivart, that a longitudinal bar supporting the cartilaginous 
rays or unpaired fins is occasionally formed ; but there is no 
trace in the embryos of the Scyllium of the bar in question 
being formed by the coalescence of rays, though the fact of 
its being perfectly continuous with the basis of the fin-rays 
is somewhat in favour of such coalescence. 

6 


66 MR. F. M. BALYOUR ON THE SKELETON 


Thacher and Mivart both hold that the pectoral and 
pelvic girdles are developed by ventral and dorsal growths 
of the anterior end of the longitudinal bar supporting the 
fin-rays. 

There is, so far as I see, no theoretical objection to be 
taken to this view; and the fact of the pectoral and pelvic 
girdles originating continuously and long remaining united 
with the longitudinal bars of their respective fins is in favour 
of it rather than the reverse. The same may be said of the 
fact that the first part of each girdle to be formed is that in 
the neighbourhood of the longitudinal bar (basipterygium) of 
the fin, the dorsal and ventral prolongations being subsequent 
growths. 

On the whole my observations do not throw much light 
on the theories of Thacher and Mivart as to the genesis of 
the skeleton of the paired fin; but, so far as they bear on 
the subject, they are distinctly favorable to those theories. 

The main results of my observations appear to me to be 
decidedly adverse to the views recently put forward on the 
structure of the fin by Gegenbaur and Huxley, both of 
whom, as stated above, consider the primitive type of fin to 
be most nearly retained in Ceratodus, and to consist of a 
central multisegmented axis with numerous lateral rays. 

Gegenbaur derives the Elasmobranch pectoral fin from a 
form which he calls the archipterygium, nearly like that of 
Ceratodus, with a median axis and two rows of rays—but 
holds that in addition to the rays attached to the median 
axis, which are alone found in Ceratodus, there were other 
rays directly articulated to the shoulder-girdle. He considers 
that in the Elasmobranch fin the majority of the lateral rays 
on the posterior (or median according to his view of the 
position of the limb) side have become aborted, and that the 
central axis is represented by the metapterygium; while the 
pro- aud mesopterygium and their rays are, he believes, 
derived from those rays of the archipterygium which 
originally articulated directly with the shoulder-girdle. 

This view appears to me to be absolutely negatived by the 
facts of development of the pectoral fin in Scylliwm—not so 


much because the pectoral fin in this form is necessarily to 


be regarded as primitive, but because what Gegenbaur holds 
to be the primitive axis of the biserial fin is demonstrated to 
be really the base, and it is only in the adult that it is con- 
ceivable that a second set of lateral rays could have existed 
on the posterior side of the metapterygium. If Gegenbaur’s 
view were correct, we should expect to find in the embryo, if 
auywhere, traces of the second set of lateral rays; but the 


a 


OF THE PAIRED FINS OF ELASMOBRANCHS. OF 


fact is that, as may easily be seen by an inspection of figs 6 
and 7, such a second set of lateral rays could not possibly 
have existed in a type of fin like that found in the embryo. 
With this view of Gegenbaur’s it appears to me that the 
theory held by this anatomist to the effect that the limbs 
are modified gill-arches also falls, in that his method of 
deriving the limbs from gill-arches ceases to be admissible, 
while it is not easy to see how a limb, formed on the type of 
the embryonic limb of Elasmobranchs, could be derived from 
a gill-arch with its branchial rays. 
Gegenbaur’s older view, that the Elasmobranch fin retains 
a primitive uniserial type, appears to me to be nearer the 
truth than his more recent view on this subject; though I 
hold the fundamental point established by the development 
of these parts in Scyllium to be that the posterior border of 
the adult Elasmobranch pectoral fin is the primitive base- 
line, 7 e. line of attachment of the fin to the side of the body. 
Huxley holds that the mesopterygium is the proximal 
piece of the axial skeleton of the limb of Ceratodus, and 
derives the Elasmobranch fin from that of Ceratodus by the 
shortening of its axis and the coalescence of some of its 
elements. The entirely secondary character of the meso- 
pterygium, and its total absence in the young embryo 
Scyllium, appear to me as conciusive against Huxley’s view 
as the character of the embryonic fin is against that of 
Gegenbaur; and I should be much more inclined to hold 
that the fin of Ceratodus has been derived from a fin like 
that of the Elasmobranchs by a series of steps similar to 
those which Huxley supposes to have led to the establishment 
of the Elasmobranch fin, but in exactly the reverse order. 
There is one statement of Davidoft’s which I cannot allow 
to pass without challenge. In comparing the skeletons of 
the paired and unpaired fins he is anxious to prove that the 
former are independent of the axial skeleton in their origin, 
and that the latter have been segmented from the axial 
skeleton, and thus to show that an homology between the 
two is impossible. In support of his view he states! that he 
has satisfied himself, from embryos of Acanthias and Scyllium, 
_ that the rays of the unpaired fins are undoubtedly products of 
the segmentation of the dorsal and ventral spinous processes. 
' This statement is wholly unintelligible to me.? From my 
examination of the development of the first dorsal and the 


1 Loe. cit., p. 514. ; ; “ 
2 Tt is possible that Davidoff may have only studied the ventral lobe of 


the caudal fin, which differs from the other unpaired fins in the fact that 
there are no interspinous elements supporting the horny fin-rays. 


68 SKELETON OF THE PAIRED FINS OF ELASMOBRANCHS. 


anal fins of Scyllium I find that their cartilaginous rays 
develop at a considerable distance from, and quite inde- 
pendently of, the neural and hemal arches, and that they are 
at an early stage of development distinctly in a more advanced 
state of histological differentiation than the neural and hzemal 
arches of the same region. I have also found exactly the 
same in the embryos of Lepidosteus. 

I have, in fact, no doubt that the skeleton of both the 
paired and the unpaired fins of Elasmobranchs and Lepi- 
dosteus is in its development independent of the axial 
skeleton. The phylogenetic mode of origin of the skeleton 
both of the paired and of the unpaired fins cannot, however, 
be made out without further investigation. 


On the Nature of the Oraan in ADULT THELEOSTEANS 
and GaNnoins, which is usually regarded as the 
Heap-Kipnuy or Pronepuros. By F. M. Batrour, 
LL.D., F.R.S., Fellow of Trinity College, Cam- 
bridge. 


Waite working at the anatomy of Lepidosteus I was led to 
doubt the accuracy of the accepted accounts of the anterior part 
of the kidneys in this and in allied species of Fishes. In order 
to test my doubts | first examined the structure of the kidneys in 
the Sturgeon (Acipenser), of which I fortunately had a well- 
preserved specimen. 

The bodies usually described as the kidneys consist of two 
elongated bands, attached to the dorsal wall of the abdomen, and 
extending for the greater part of the length of the abdominal cavity. 
In front each of these bands first becomes considerably narrowed, 
and then expands and terminates in a great dilatation, which is 
usually called the head-kidney. Along the outer border of the 
hinder part of each kidney is placed a wide ureter, which ends 
suddenly in the narrow part of the body, some little way behind 
the head-kidney. To the naked eye there is no distinction in 
structure between the part of the so-called kidney in front of 
the ureter and that in the region of the ureter. Any section 
through the kidney in the region of the ureter suffices to show 
that in this part the kidney is really formed of uriniferous 
tubuli with numerous Malpighian bodies. Just in front, how- 
ever, of the point where the ureter ends the true kidney sub- 
stance rapidly thins out, and its place is taken by a peculiar 
tissue formed of a trabecular work filled with cells, which I 
shall in future call lymphatic tissue. Thus the whole of that 
— part of the apparent kidney in front of the ureter, including the 
whole of the so-called head-kidney, is simply a great mass of 
lymphatic tissue, and does not contain a single uriniferous tubule 
or Malpighian body. 

The difference in structure between the anterior and posterior 
parts of the so-called kidney, although not alluded to in most 
modern works on the kidneys, appears to have been known to 


70 F. M. BALFOUR. 


Stannius, at least I so interpret a note of his in the second 
edition of his ‘Comparative Anatomy,’ p. 263, where he 
describes the kidney of the Sturgeon as being composed of two 
separate parts, viz. a spongy vascular substance (no doubt the 
so-called head-kidney) and a true secretory substance. 

After arriving at the above results with reference to the 
Sturgeon I proceeded to the examination of the structure of the 
so-called head-kidney in Teleostei. 

I have as yet only examined four forms, viz. the Pike (Hsoxr 
lucius), the Smelt (Osmerus eperlanus), the Hel (Anguilla 
anguilla), and the Angler (Lophius piscatorius). 

The external features of the apparent kidney of the Pike 
have been accurately deseribed by Hyrtl.! He says: ‘‘ The 
kidneys extend from the second trunk vertebra to the end of 
the abdominal cavity. Their anterior extremities, which have 
the form of transversely placed coffee beans, are united together, 
and lie on the anterior end of the swimming bladder. The con- 
tinuation of the kidney backwards forms two small bands, sepa- — 
rated from each other by the whole breadth of the vertebral 
column. They gradually, however, increase in breadth, so that 
about the middle of the vertebral column they unite together 
and form a single symmetrical, keel-shaped body,” &c. 

The Pike I examined was a large specimen of about 58 
centimétres in length, and with an apparent kidney of about 254 
centimétres. The relations of lymphatic tissue and kidney 
tissue were much as in the Sturgeon. The whole of the ante- 
rior swelling, forming the so-called head-kidney, together with a 
considerable portion of the part immediately behind, forming 
not far short of half the whole length of the apparent kidney, 
was entirely formed of lymphatic tissue. The posterior part of 
the kidney was composed of true kidney substance, but even at 
16 centimétres from the front end of the kidney the lymphatic 
tissue formed a large portion of the whole. 

A rudiment of the duct of the kidney extended forwards for a 
short way into the lymphatic substance beyond the front part of 
the functional kidney. 

In the Smelt (Osmerus eperlanus) the kidney had the typical 
Teleostean form, consisting of two linear bands stretching for 
the whole length of the body-cavity, and expanding into a great 
swelling in front on the level of the ductus Cuvieri, forming the 
so-called head-kidney. The histological examination of these 
bodies showed generally the same features as in the case of the 
Sturgeon and Pike. The posterior part was formed of the 
usual uriniferous tubuli and Malpighian bodies. The anterior 


1 “ Das Uropoétische System d. Knochenfische,” Sitz. d. ‘ Wien, Akad.,’ 
1850. 


HEAD-KIDNEY IN ADULT TELEOSTEANS AND GANOIDS. ZS 


swollen part of these bodies, and the part immediately follow- 
ing, were almost wholly formed of a highly vascular lymphatic 
tissue ; but in a varying amount in different examples portions 
of uriniferous tubules were present, mainly, however, in the 
region behind the anterior swelling. In some cases I could 
find no tubules in the lymphatic tissue, and in all cases the 
number of them beyond the region of the well-developed part of 
the kidney was so slight, that there can be little doubt that they 
are functionless remnants of the anterior part of the larval kidney. 
Their continuation into the anterior swelling, when present, con- 
sisted of a single tube only. 

In the Keel (Anguilla anguilla), which, however, I have not 
examined with the same care as the Smelt, the true excretory 
part of the kidney appears to be confined to the posterior portion, 
and to the portion immediately in front of the anus, the whole 
of the anterior part of each apparent kidney, which is not 
swollen in front, being composed of lymphatic tissue. 

Lophius piscatorius is one of the forms which, according to 
Hyrtl,! is provided with a head-kidney only, z.¢. with that part 
of the kidney which corresponds with the anterior swelling of 
the kidney of other types. Jor this reason I was particularly 
anxious to investigate the structure of its kidneys. 

Hach of these bodies forms a compact oval mass, with the 
ureter springing from its hinder extremity, situated in a for- 
ward position in the body cavity. Sections through the kidneys 
showed that they were throughout penetrated by uriniferous 
tubules, but owing to the bad state of preservation of my speci- 
mens I could not come to a decision as to the presence of Mal- 
pighian bodies. The uriniferous tubules were embedded in 
lymphatic tissue, similar to that which forms the anterior part 
of the apparent kidneys in other Teleostean types. 

With reference to the structure of the Teleostean kidneys, the 
account given by Stannius is decidedly more correct than that 
of most subsequent writers. In the note already quoted he gives 
it as his opinion that there is a division of the kidney into the 
same two parts as in the Sturgeon, viz. into a spongy vascular 
part and a true secreting part; and ona subsequent page he 
points out the absence or poverty of the uriniferous tubules in 
the anterior part of the kidney in many of our native Fishes. 

Prior to the discovery that the larve of Teleosteans and 
Ganoids were provided with two very distinct excretory organs, 
viz. a pronephros or head-kidney, and a mesonephros or Wolffian 
body, which are usually separated from each other by amore or 
less considerable interval, it was a matter of no very great im- 


1 “ DasUropoetische System de Knochenfische,” ‘ Sitz, d. Wien. Akad.’ 
1850. 


72 F. M. BALFOUR. 


portance to know whether the anterior part of the so-called — 
kidney was a true excretory organ. In the present state of 
our knowledge the question is, however, one of considerable 
interest. | 

In the Cyclostomata and Amphibia the pronephros is a purely 
larval organ, which either disappears or ceases to be functionally 
active in the adult state. 

Rosenberg, to whom the earliest satisfactory investigations on 
the development of the Teleostean pronephros are due, stated 
that he had traced in the Pike (Hsox ductus) the larval organ into 
the adult part of the kidney, called by Hyrtl the pronephros ; 
and subsequent investigators have usually assumed that the so- 
called head-kidney of adult Teleosteans and Ganoids is the 
persisting larval pronephros. 

We have already seen that Rosenberg was entirely mis- 
taken on this point, in that the so-called head-kidney of the 
adult is not part of the true kidney. From my own studies 
on young Fishes I do not believe that the oldest larvee imvesti- 
gated by Rosenberg were sufficiently advanced to settle the point 
in question; and, moreover, as Rosenberg had no reason for 
doubting that the so-called head-kidney of the adult was part 
of the excretury organ, he does not appear to have studied the 
histological structure of the organ which he identified with the 
embryonic pronephros in his oldest larva. 

The facts to which I have called attention in this paper 
demonstrate that in the Sturgeon the larval pronephros un- 
doubtedly undergoes atrophy before the adult stage is reached. 
The same is true for Lepidosteus, and may probably be stated for 
Ganoids generally. 

My observations on Teleostei are clearly not sufficiently exten- 
sive to prove that the larval pronephros zever persists in this 
group. ‘They appear to me, however, to show that in the normal 
types of Teleostei the organ usually held to be the pronephros is 
actually nothing of the kind. 

A different interpretation might no doubt be placed upon my 
observations on Lophius piscatorius, but the position of the 
kidney in this species appears to me to be far from affording a 
conclusive proof that it is homologous with the anterior swelling 
of the kidney of more normal Teleostei. 

When, moreover, we consider that Lophius, and the other 


forms mentioned by Hyrtl as being provided with a head-kidney _ 


only, are all of them peculiarly modified and specialised types of 
Teleostei, it appears to me far more natural to hold that their 
kidney is merely the ordinary Teleostean kidney, which, like 
many of their other organs, has become shifted in position, than 
to maintain that the ordinary excretory organ present in other 


HEAD-KIDNEY IN ADULT TELEOSTEANS AND GANOIDS., 73 


Teleostei has been lost, and that a larval organ has been retained, 
which undergoes atrophy in less specialised Teleostei. 

As the question at present stands, it appears to me that the 
probabilities are in favour of there being no functionally active 
remains of the pronephros in adult Teleostei, and that in any 
ease the burden of proof rests with those who maintain that 
such remnants are to be found. 

The general result of my investigations is thus to render it 
probable that the pronephros, though found in the larve or em- 
bryos of almost all the Ichthyopsida, except the Elasmobranchit, 
is always a purely larval organ, which never constitutes an active 
part of the excretory system in the adult state. 

This conclusion appears to me to add probability to the view 
of Gegenbaur that the pronephros is the primitive excretory 
gland of the Chordata; and that the mesonephros or Wolffian 
body, by which it is replaced in existing Ichthyopsida, is phylo- 
genetically a more recent organ. 

In the preceding pages I have had frequent occasion to allude 
to the lymphatic tissue which has been usually mistaken for 
part of the excretory organ. This tissue is formed of trabecular 
work, like that of lymphatic glands, in the meshes of which an 
immense number of cells are placed, which may fairly be com- 
pared with the similarly placed cells of lymphatic glands. In 
the Sturgeon a considerable number of cells are found with 
peculiar granular nuclei, which are not found in the Teleostei. 
In both groups, but especially in the Teleostei, the tissue is 
highly vascular, and is penetrated throughout by a regular 
plexus of very large capillaries, which appear to have distinct 
walls, and which pour their blood into the posterior cardinal 
vein as it passes through the organ. The relation of this tissue 
to the lymphatic system I have not made out. 

The function of the tissue is far from clear, Its great 
abundance, highly vascular character, and presence before the 
atrophy of the pronephros, appear to me to show that it cannot 
be merely the non-absorbed remnant of the latter organ. From 
its size and vascularity it probably has an important function ; 
and from its structure this most either be the formation of lymph 
corpuscles or of blood corpuscles. 

In structure it most resembles a lymphatic gland, though, 
till it has been shown to have some relation to the lymphatic 
system, this can go for very little. 

On the whole, I am provisionally inclined to regard it as a 
form of lymphatic gland, these bodies being not otherwise 
represented in fishes. 


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EXPLANATION OF PLATE VI, 


Illustrating Mr. K. Mitsukuri’s Paper on the “ Develop- 
ment of the Suprarenal Bodies in Mammalia.” 


Explanation of Figures. 


The outlines of all the figures, except Fig. 10, are drawn with Zeiss’s 
obj. A and camera lucida with eye-piece 4, and then reduced one-third, 
ant Fig. 1. Fig .10 is drawn with Zeiss’s obj. a and the same camera 
ucida. 

General Letters of Reference. 


Ao. Aorta. c. Cortical part of the suprarenal bodies. ch. Notochord. 
m. Medullary part of the suprarenal. . Peripheral sympathetic part with 
ganglion cells. g. Peripheral sympathetic part without ganglion cells. 
p.p. Body cavity. - s.r. Suprarenal bodies. symp. Sympathetic cords. 
eee a v.c. Cardinal veins. v.c.é. Vena cava inferior. W.6. Wolffian 

ody. 


Fic. 1.—A part of the transverse section of the adult suprarenal body of 
the rabbit. 
d, Capsule. a. The outermost zone of the cortex. 4. Zona fasci- 
culata. c. Zona reticulatis. w.w. Blood-cavities. 


Fic. 2.—A longitudinal section of the posterior end of the adult supra- 
renal body of the rabbit. The upper end of the figure is posterior. 

a. Cells like ganglion cells. 4. A place where one of the irregular 
cords of cells taper and seem to pass off into nerve fibres. d. Blood- 
capillaries. g. Ganglion-like mass. 4%. Blood-vessel. y. Nerve- 
bundle. 

Fie. 3.—Section from a rabbit embryo twelve days old. 


Fie, 4.—Section from a rabbit embryo fourteen days old. 
g- Germinal band. 


Fic. 5.—Section from a rabbit embryo sixteen days old. 
a. Nervous fibres given off from the mass z. 


Fic. 6.—Section immediately behind that represented in Fig. 5 from the 
same embryo. 


Fic. 7.—Section of the right suprarenal from a rat embryo 23 mm. long. 
6. Nervous bundles entering the suprarenal. 
Fig. §.—Section of the suprarenal from an embryo rabbit twenty-six days 
old, near the middle of the organ. 
Fic. 9.—Ditto, near the posterior end. The outer part of the suprarenal 
is torn away. 
a. Nervous bundle. 
Fic. 10.—Series of diagrammatic longitudinal sections of the suprarenal 
from an embryo rabbit twenty-four days old. 
A is the furthest from and D the nearest to the median axis of the 
body of the embryo. Between A and B there is one section. B and 
C are consecutive. Between C and D there are two sections. In 
each figure the upper end is anterior and the right side dorsal. 


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PLATE Ml 


poet 
Sate 


On the Devetopment of the SuprarENaL Bopizs in- 
Mamaia. By K. Mirsuxvurti, Ph.B., University 
of Tokio, Japan. 


(With Plate VI.) 


Ir has for a long time been known that the suprarenal 
bodies are in close connection with the sympathetic system. 
As far back as 1839 Bergmann is said to have noted the 
fact. Remak, writing in 1847, clearly stated the relation 
or embryological grounds, and even went so far as to call 
the suprarenal bodies “‘ Nervendrisen.” Leydig! is explicit 
in his statements in regard to the point. According to him, 
the suprarenal bodies in Selachians, Ganoids, and Reptiles, 
consist of two distinct parts. ‘The one is enclosed, together 
with the usual ganglion cells, in successive sympathetic 
ganglia, and is made up of masses of cells different from the 
ganglion cells only in being dirty yellow in colour; the other 
part is derived from the former by the deposition of fat globules 
in the cells, and is situated on blood-vessels as yellowish 
masses. He further maintained that the ganglionic part 
corresponds to the medullary, and the yellowish masses to 
the cortical part of the Mammalian (and Avian ?) suprarenal 
bodies, which, in these classes, are coalesced into one mass 
on each side. According to this view the cortical part of the 
suprarenal bodies of the higher types must, therefore, be de- 
rived from the medullary—a conclusion which will be shown 
to be erroneous. It should be mentioned that in an earlier 
work? Leydig did not believe in the derivation of the one part 
from the other—in fact, considered the yellowish masses 
simply as collections of fat cells. 

Balfour, in his ‘Monograph on the Development of Elas- 
mobranch Fishes,’ mentions ‘‘ two structures that have gone 
under the name of the suprarenal bodies.” The one called 
by him the interrenal body is “an impaired rod-like body, 

1 ‘Anatomisch-Histologisch Untersuchungen iiber Fische und Rep- 
tilien,” Berlin; 1853, and ‘ Lehrbuch der Histologie des Menschen und 


der Thiere.’ 
2 *Beitrage zur Mikros. Anat. &c., der Rochen und Haie,’ Leipzig, 


1852, 


76 K. MITSUKURI. 


lying between the dorsal aorta and the caudal vein, in the 
region of the posterior end of the kidneys.” The other, 
named the suprarenal bodies, consists of “a series of paired 
bodies, situated dorsal to the cardinal veins on the branches 
of the aorta, and arranged segmentally.” The former was 
believed by him to be developed from the mesoblast, and to 
have nothing to do with the latter, which are formed out of 
sympathetic ganglia, and remain in close connection with 
them throughout life. At that time Mr. Balfour was inclined 
to consider the suprarenal bodies homologous with the organ 
that goes under that name in the higher vertebrates, and the 
interrenal body, an altogether independent structure. He 
therefore agreed, to some extent, with the earlier view of 
Leydig. It will be seen that he has since modified his 
opinions. 

Braun! studied the anatomy and development of the supra- 
renal bodies in Reptilians. In that class they are formed of 
two parts, viz. (1) masses of brown cells placed on the dorsal 
side of, and closely applied against, (2) irregular cords of 
cells, so full of oil globules that nuclei are altogether 
invisible in the fresh state. The first is derived from the 
sympathetic ganglia, and the second from the ordinary 
mesoblast. Here, clearly, the nervous cells, having the cha- 
racteristic reaction of being stained brown by bichromate of 
potash, are homologous with the medullary, and the irregular 
cords containing oil globules, with the cortical part of the 
Mammalian suprarenals. 

Brunn® makes an interesting statement in regard to the 
suprarenal bodies of birds. According to him the cells 
stained brown by bichromate of potash—. e. the elements 
composing the medullary part of the mammalian suprarenals 
—are scattered throughout the organ between irregular cords 
of cells, which are like those composing the cortical part of 
the higher type. 

Summing up these observations Mr. Balfour says, in his 
‘Comparative Embryology’ (vol. ii, pp. 548-9), “The 
structure and development of what I have called the in- 
terrenal body in Elasmobranchii so closely correspond with 
that of the mesoblastic part of the suprarenal bodies of the 
Reptilia, that I have very little hesitation in regarding them 
as homologous ; while the paired bodies in Elasmobranchii, 


1M. Braun,—* Bau u. Entwick. d. Nebennieren bei Reptilien,” 
‘ Arbeit a. d. zool.-zoot. Institut, Wurzburg,’ vol. v, 1879. 

2? A. von Brunn.—“ Ein Beitrag zur Kentniss des feineren Baues und 
Entwicklungsgeschichte der Nebennieren,” ‘Archiv fiir mikros. Anat.,’ 
vol. viii, 1872. 


DEVELOPMENT OF SUPRARENAL BODIES IN MAMMALIA. an 


derived from the sympathetic ganglia, clearly correspond 
with the part of the suprarenals of Reptilia, having a similar 
origin, although the anterior parts of the paired suprarenal 
bodies of Fishes have clearly become aborted in the higher 
types. 

“In Elasmobranch Fishes we thus have (1) a series of 
paired bodies derived from the sympathetic ganglia, and (2) 
an unpaired body of mesoblastic origin. In the Amniota 
these two bodies unite to form the compound suprarenal 
bodies, the two constituents of which remain, however, dis- 
tinct in their development. The mesoblastic constituent 
appears to form the cortical part of the adult suprarenal 
body, and the nervous constituent the medullary part.” 

In view of these considerations it seemed worth while to 
trace the development of the suprarenal bodies in Mammalia, 
and see whether the medullary part is actually derived from 
the sympathetic system and the cortical part from the me- 
soblast.. Accordingly, at the instance of Mr. Balfour, I have 
been engaged in following the history of these bodies in the 
rabbit, and to some extent in the rat. The result has fully 
justified the conclusions set forth by Mr. Balfour in the 
above quotations. It will be shown in this article that the 
medullary part of the Mammalian suprarenals arise from 
the sympathetic nervous system, totally independently of and 
outside the mesoblastic cortical part, and becomes, in the 
course of development, transported into the middle of the 
cortical part, gaining only then the position which its name 
implies. . 

Before proceeding to describe the successive stages of 
development it may, however, be well to recall here briefly 
the essential points in the structure of the adult suprarenal 
bodies in Mammalia. The account given below will be 
understood to refer to the rabbit, unless otherwise specified. 

Structure of the Adult Suprarenal Bodies—In the rabbit 
the suprarenals lie, usually in a large quantity of fat, at some 
distance from the kidneys, near the opening of the renal vein 
into the vena cava inferior, and about on a level with the ante- 
rior end of the kidney of their respective side. The right is, 
therefore, somewhat in front of the left, and is, moreover, 
on the dorsal side of the vena cava, while the left is rather 
on the ventral side. The suprarenals themselves are oval, 
reniform, or elongated in shape, and lie with their longest 
diameter parallel with the axis of the body. ‘The right 
suprarenal is often more elongated than the left, which is 
generally oval or reniform, in the latter case having its con- 
cave side turned towards the median line of the body. A 


78 K. MITSUKURI, 


large vein is seen to enter each suprarenal at its posterior end, 
which is a branch of the renal vein in the case of the left, 
and of the vena cava inferior in the right suprarenal. On 
cutting open a fresh suprarenal, in a median longitudinal 
plane, its division into whitish-yellow cortical part and 
greyish medullary part becomes at once obvious. The 
former exhibits also a subdivision into three more or less 
distinct layers. Starting from the outside, they are (1) a 
rather thin greyish layer; (2) yellowish layer comprising 
the main mass of the organ, and showing, even to the naked 
eye, radial striation; and (3) a layer, of a much darker 
yellow, adjoining to the medullary substance. These layers 
are for the most part arranged concentrically ; but at the 
posterior end there is a modification in their relations, which, 
singularly enough, seems to have hitherto escaped observa- 
tion. The medullary substance, instead of being covered 
by the cortical layers, as in other parts, here becomes 
attenuated into a narrow streak, and reaches the outside. 
Roughly speaking, therefore, the cortical substance is in the 
shape of a horseshoe, completely surrounding the medullary 
part, except at one point at the posterior end. The histo- 
logical structure of this part is of great interest, as will be 
seen further on, on account of the peculiar developmental 
history of the medullary substance. A section of the supra- 
renal body across its shorter diameter will show three layers 
in concentric rings. 

Fig. 1 shows a part of a transverse section of the supra- 
renal considerably magnified. Ifthe whole section had been 
figured the outline would be oval, and the medullary sub- 
stance (m), of which only a small portion is represented, 
would occupy a rather large irregularly oval area in the 
centre. In the figure, however, are shown all three parts 
of which the suprarenal is composed, viz. (1) the outer cap- 
sule of connective tissue (d), (2) the cortical substance 
(a,b, c), and (3) the medullary substance (m). I shall 
briefly describe each of these three portions. 

Of the connective tissue capsules I need only say that 
nerves and blood-vessels are found embedded in it in toler- 
able abundance, and that bundles of connective-tissue fibres 
from the capsule are sent inwards to form the framework 
of the whole organ. 

The whole space between the outer capsule and the medul- 
lary part is occupied by the cortical substance, which, there- 
fore, constitutes the main mass of the organ. Briefly speaking, 
it is made up of large cells, supported in a very fine network 
of connective tissue—so fine that each cell has its own cavity 


DEVELOPMENT OF SUPRARENAL BODIES IN MAMMALIA. 79 


inthe mesh. These cells are collected into groups by coarser 
trabeculz, and the forms these cell groups assume in different 
layers give characteristic appearances to any particular layer. 
We thus recognise in the suprarenals of the rabbit three 
distinct zones (a, 6, ce, fig. 1), which correspond to the 
three layers visible to the naked eye (see above). The outer- 
most zone (@) shows cell groups in long radiating columnar 
rows, which, directly under the capsule, form small arches, 
as shown in the figure. The cells are more closely packed 
than in the middle zone. Between the columns are rather 
large, longish spaces (w, w), which are shown to be sections 
of blood-vessels by the endothelial cells lining them. This 
zone gradually passes into the inner (0), where the columns 
are shorter and thicker, and cells are not as closely packed. 
This zone forms, in most sections, by far the largest portion 
of the cortical substance. Its cells stand out beautifully 
Between the columns there may be seen, running radially, fine 
capillary blood-vessels, shown to be such by their endothelial 
cells. The innermost zone (c) has irregular cell groups, and 
is characterised by the presence of a great many blood-vessels 
(v, v, v). The latter, becoming larger and larger toward the 
centre by the union of numerous branches, finally open into 
the central large veins, found in the medullary substance, 
which, in their turn becoming one, leave the organ at its 
posterior end. 

I may remark in passing that the two inner zones men- 
tioned above correspond to the zona reticularis and zona 
fasciculata of Arnold, but the outermost is different from 
his zona glomerulosa, as the cell groups are oval in the 
latter. 

In sections cut with a freezing microtome from a fresh 
suprarenal, the whole cortical part is so filled with fat-like 
globules that it is hard to see anything else. ‘These fat-like 
globules are not stained by osmic acid, and therefore do not 
seem to be true fat. In the process of hardening in picric 
and chromic acids and other reagents, except alcohol, they 
disappear completely. Even with specimens hardened in 
alcohol they seem to be lost in the process of embedding in 
wax. 

The medullary cells are collected into oval or roundish 
groups by network of connective tissue, and I have not suc- 
ceeded in seeing any one cell distinctly. The groups look 
like a collection of nuclei embedded in a mass of protoplasm. 
This alone would separate them from the cortical cells, each 
of which, as has been remarked, is placed in a special mesh 

1 * Virchow’s Archiv,’ 1866, 


80 K. MITSUKURI. 


of connective tissue, and stands out distinctly. There is, 
however, a reaction very characteristic of the medullary cells 
which is most useful in identifying them. When the supra- 
renals are hardened in bichromate of potash, the medullary 
substance is stained brown or yellow, and is sharply marked 
off from the cortical part. It occupies an area in the centre 
corresponding roughly to the outline of the section. The 
boundary between it and the cortical part is, however, most 
irregular, the latter sometimes going into the midst of the 
former, which, in its turn, may send long processes out- 
wards. In the last case we find almost invariably that the 
processes become continuous with nerve fibres, which fre- 
quently traverse the cortical substance from the capsule 
toward the medulla. 

Different authors have stated that the medullary part of 
various animals is very rich in ganglion_cells. In the rabbit 
they are very scarce. Out of the numerous sections I have 
cut there is only one that shows undoubted nerve cells in 
the medulla. Large veins are found in this part (see fig. 1), 
and capillary spaces (d, fig. 2) are visible between cell 
groups. 

Reference has been made to the posterior end of the 
suprarenal, where the medullary substance reaches the out- 
side of the organ. By studying the consecutive sections of 
this part, we find the medullary cells following the vein to 
the posterior part, and finally, near the exit of the latter, 
emerging on the exterior. In the suprarenals of the fully- 
grown rabbit, the band of cells which follows the vein is 
rather narrow, while in those of young examples it is as 
wide as any part of the medullary substance. Fig. 2 is a 
longitudinal section of the posterior end of the suprarenal of 
an adult. The upper end of the figure is the posterior 
extremity of the suprarenal. The areas marked e, ¢, c, are 
occupied by cortical cells. If we follow the medullary sub- 
stance (m m) a few sections towards the median plane of 
the suprarenal, we should find the point (x) gradually 
approaching the middle line of the section, and finally 
uniting with the main mass of the medullary substance in 
the centre of the organ. It will be seen in the figure that 
the medullary substance emerges at two points—on the 
top, aud on the Teft side—and spreads itself over more or 
less wide areas on the surface of the suprarenal. At fit 
becomes continuous with irregular cords (h), made up of 
cell groups, which are exactly like those of the medulla, 
and some of which are stained: yellow by bichromate of 
potash. They are, in fact, a part of the medulla. . These 


DEVELOPMENT OF SUPRARENAL BODIES IN MAMMALIA, 81 


cords (2) are continuous with the mass (g). This contains, 
in addition to small cells like those of the medullary sub- 
stance, large, well-defined cells (a), with nuclei much larger 
than those of the surrounding cells. They seem to me to 
be nerve cells. In the sections of the corresponding part 
from a rabbit three weeks old I have seen an undoubted 
large ganglion on one side of the section, and I am almost 
certain that it becomes continuous with irregular cords, con- 
tinuous in their turn with the medulla. In many places, 
as 0, these cords taper and seem to pass off into fibres, which 
are certainly like nerve fibres, and at others, as y, nerves 
pass into cell masses, and are lost among them. 

Taking these facts in connection with the developmental 
history of the medulla, it seems to me almost certain that 
the process of medullary substance emerging at the outer 
surface of the suprarenal at its posterior end becomes in 
some way intimately connected with nerves and ganglion 
cells. J may remark that Braun (loc. cit.) finds ganglia at 
the anterior and posterior terminations of the suprarenals in 
the Reptiles, and that the medullary cells are in intimate 
connection with them. My researches appear to show that 
the same arrangement obtains in the rabbit at the posterior 
end. 

In the human suprarenals, according to ‘Quain’s Anatomy,’ 
the arteries are derived from the aorta, the phrenic and the 
renal arteries. Brunn (loc. cit.) remarks that many arteries 
entering the connective-tissue capsule divide themselves in 
it into numerous fine branches; some of these go directly 
through the cortical substance to the medulla, but by far 
the majority form a close anastomosis in the capsule, and 
enter the suprarenal with larger process of connective tissue. 
The fine network of capillaries in the cortical and medullary 
part has already been mentioned. In regard to nerves, I 
am only certain of the existence of one in the rabbit arising 
from the sympathetic cords far in front in the dorsal region, 
and entering the suprarenal just within its posterior half. 
In ‘Quain’s Anatomy’ nerves are stated to be derived from 
the solar plexus of the sympathetic and from the renal 
plexuses, and to be exceedingly numerous. 

Development of the Suprarenal Bodies.— According to 
Kolliker,! the suprarenal bodies in the rabbit appear first 
on the twelfth or thirteenth day of gestation as masses of 
somewhat large round cells on each side of, and ventral to, 
the aorta, on the inner side of the Wolffian bodies, and 
dorsal to the mesentery. 

1 «Entwicklungsgesch. des Menschen und der Hoheren Thiere,’ p. 954. 

7 


82 K. MITSUKURI. 


In one series of my sections of a twelve-day embryo several, 
that pass through what must become the front part of the 
future abdomen, show, at the spot designated by Kolliker, a 
mass of cells (s.7., fig. 3) with large nuclei, stained with hema- 
toxylin slightly darker than those of the surrounding cells. 
Dorsally this mass is tolerably distinctly separated off from 
other mesoblastic cells, but ventrally its termination is in- 
definite. This mass is probably the first rudiment of the 
mesoblastic or cortical part of a suprarenal body. 

On the fourteenth day the suprarenals are already well 
marked. Fig. 4 shows the left suprarenal (s. 7.) a few 
sections behind the front end. It consists of a mass of cell 
with large nuclei, divided into indefinite cords by blood 
capillaries which are already somewhat numerous. Cell 
limits are hardly visible. Mesoblastic cells surround the 
mass as a capsule. Dorsal to the suprarenal is placed 
another mass of cells (7) looking very much like the former, 
but if anything, with slightly smaller nuclei stained darker. 
This, on tracing it forward, is found continuous with the sym- 
pathetic cells (symp., fig. 4), although at this stage the con- 
nection is not so obvious as later on. 

The subsequent history shows that the medullary sub- 
stance arises out of the above ventral mass (”) of the sym- 
patheticsystem. From following the sections the suprarenal 
is found to be widest in the middle, and to taper both in 
front and behind. ‘The cords of which it is composed are 
tolerably distinct, and the blood channels between them pro- 
portionally very wide and conspicuous. The sympathetic 
mass (7) gradually extends ventrally between the aorta and 
the suprarenal, and is continued back far beyond the poste- 
rior end of the suprarenal. It spreads, in some sections, 
around the latter, and in such cases it is difficult to tell the 
two bodies apart, as their structure at this early stage is 
very similar. It is only by tracing them back that they can 
be distinguished. The suprarenal and the sympathetic 
masses of the two sides remain, however, separate 
throughout. 

In the suprarenals of sixteen-day embryos, great 
changes are observable. The medullary part (m, fig. 5) 
surrounded for the most part by the cortical substance (c) 
can now be clearly distinguished. In the medulla the nuclei 
of the cells are stained darker than in the surrounding 
part, and the medulla itself is at this stage nothing but a 
mass of simple cells with a mixture of a great number of 
spindle cells and other connective-tissue elements. The corti- 
cal substance is still made up of irregular cords of cells without 


DEVELOPMENT OF SUPRARENAL BODIES IN MAMMALIA. 88 


any more definite structure. Closely applied against the supra- 
renal, on its inner side, there is a large mass of sympathetic 
nerve cells (7). At its ventral end, a process of this mass, 
partly composed of nerve fibres, enters the cortical substance 
at the point (a). The nervous mass has very much the appear- 
ance of the medullary substance—its nuclei being stained in 
the same way—although there are no connective-tissue cells 
visible in it. On tracing it forward it becomes continuous 
with the main sympathetic cord dorsal to the aorta. Fig. 6 
shows a section immediately behind that represented in 
fig. 5. Here we see the branch (a, fig. 5) of the nervous 
mass () passing into the suprarenal and uniting with the 
medullary substance (m), a relation which affords strong 
presumptive evidence that the latter is derived from the 
nervous mass without. To demonstrate this point. still 
more conclusively I have introduced here a figure (fig. 7) of a 
section of the suprarenal of an embryo rat about 23 milli- 
métres long. In this figure there are to be seen. nerve 
fibres () starting from the nervous mass (7) and entering the 
suprarenal (s.r). Amongst the fibres numerous cells (m), 
that look exactly like those in the mass (7),! have found their 
way into the middle of the suprarenal, and are clearly dis- 
tinguishable from the surrounding cortical cells (c, c) as well 
by the structure of the cell groups as by their darker stain- 
ing. In several sections, continuous with the one figured, 
strands of nerve fibres dividing from the main bundle may be 
seen proceeding into various parts of the suprarenal, and 
wherever they go, they are followed by nerve cells. That 
these cells become the medullary substance there can be 
no doubt, as I have seen the parts occupied by them stained 
brown by bichromate of potash in somewhat older rabbit 
embryos. I ought to add that the mass (7) is continuous 
with the main sympathetic cords. 

' As we trace the sections of the sixteen-day embryo 
rabbits backward, we find the nervous mass (n, figs. 5 and 6) 
gradually spreading towards the ventral side of the aorta, 
and there constituting what must be regarded as a sympa- 
thetic plexus. This plexus seems to join the medulla of the 
suprarenal at more than one point. The medullary sub- 
stance continues to the posterior end of the suprarenal, 
occupying irregular areas more or less in the centre. 
Towards the posterior termination it occupies by far the 
larger portion of a section of the suprarenal, and is covered 


1 The engraver has unfortunately not represented the nuclei of the mass 
(~) very satisfactorily ; so that their similarity with those of the medullary 
substance is not obvious. 


84 K. MITSUKURI. 


by the cortical substance only on the outer side. The cortex 
finally disappears altogether, but the continuations of the 
medullary substance of the two suprarenals proceed back- 
wards as paired cords along the ventral side of the aorta. 
These are evidently the “‘ Geschlechtsnerven”’ of Remak, 
from the anterior part of which the suprarenals are stated 
by him to be developed. These cords join the continuation 
of the nervous mass (”, figs. 5 and 6) just at the point 
where the cortex ceases. The cords seem to unite with each 
other at one point, although soon separating again. Professor 
Kolliker (loc. cit. p. 953) remarks that in the sixteen- 
and seventeen-day embryo rabbits, the suprarenals of two 
sides unite behind, while the front parts are separate. It 
seems evident to me that he has not clearly distinguished © 
between the cortical and medullary part. The cortical parts of 
two sides certainly show no signs of uniting with each other at 
any stage of development, while, if reference is made only 
to the continuations of the medullary part, it is not only in 
the sixteenth toseventeenth day, but at much later stages that 
they are found uniting with each other. Of certain histo- 
logical peculiarities of other cords I shall speak later on. 

The method of entrance of the medullary substance into 
the suprarenal bodies may be stated briefly as follows :—The 
peripheral sympathetic plexus, which is formed ventrally to 
the aorta in the abdominal region, sends in processes into 
the body of suprarenals at various points—the one at the 
posterior end of the organ being by far the largest—and the 
cells thus carried in become gradually transformed into the 
cells of the medullary substance. 


In embryos of sixteen days, all the essential parts of the 
suprarenals are already present, and henceforth the develop- 
ment consists simply in their histological elaboration :—I 
shall rapidly describe the successive stages until we come to 
the twenty-sixth day, which, being the last I have observed, 
I shall treat of somewhat minutely. On the eighteenth 
day the suprarenals are already visible to the naked eye 
as oval bodies on the inner side of the kidney, and substan- 
tially resemble the adult bodies in regard to position, shape, 
and symmetry. This stage is the earliest of which I have 
embryos hardened in bichromate of potash. 

The medullary part takes already a slight brown staining, 
and itis to be specially noted that the continuation of it 
behind the cortical substance is also affected in the same 
way. The cortical substance has increased considerably in 
quantity, and the irregular cords of cells begin to assume a 


DEVELOPMENT OF SUPRARENAL BODIES IN MAMMALIA. 85 


more regular oval or polygonal form. In regard to the em- 
bryos of the twentieth, twenty-second, and twenty-fourth days, 
of which I have sections, there is nothing special to mention, 
except that the suprarenals become gradually larger, that the 
medullary and cortical substances increase accordingly, and 
that the cell groups in the cortex become more and more defi- 
nite and the trabecule between them finer. We now come to 
the twenty-six-day embryos. Fig. 8 shows a transverse 
section of the left suprarenal (s. 7.) about in the middle of 
the organ. The cortex (c,c) is already made up of definite 
cell groups, although not yet divided into the different zones 
found in the adult. The connective-tissue framework is now 
so fine that each cell has its own special mesh, although not 
so represented in the figure. Capillaries between the cell 
groups are well formed. The differentiated medullary sub- 
stance is not so far advanced as that of the cortical part; it 
is divided into irregular groups of cells, whose nuclei are 
stained darker than those of the cortex. Its veins (v, v) are 
very conspicuous. The connective-tissue capsule is well 
developed. 

The sympathetic nervous masses (”, 7) are now full of 
distinct ganglion cells, supported in a connective-tissue 
network. Scattered among the larger cells are smaller cells, 
as ata. Fig. 9 shows a section of the same suprarenal near 
its posterior end. It will be seen that the cortex (c,c) no 
longer completely surrounds the medullary part, but is open 
toward the inner side. If we trace it backward we shall find 
it occupying less and less space in the section of the supra- 
renal, and gradually confining itself to the outer side. 
Finally, having greatly diminished in quantity, it separates 
from the medullary part, and soon after ends on the ventral 
side of the vena cava inferior. The diagrammatic longitudinal 
section of the same part, shown in fig. 10 A, will make the 
relations clear. The cortical substance (c) is present only 
on the ventral side at the posterior end of the suprarenal ; 
and, if it were possible to show this in the same figure, would 
be also visible on the outside; while the medullary substance 
(m.) is confined to the dorsal and inner region. Certain 
parts of the medullary substance present very peculiar 
features. I refer to the parts marked p, pin fig. 9. In 
them, spindle and stellate connective-tissue cells are very 
abundant, and, by the union of their processes, seem to 
divide the whole space into irregularly polygonal areas. In 
these areas there are placed a number of small cells, some- 
what like those of the central part of the medullary sub- 
tance, with which the part p, p is directly continuous. It 


86 K. MITSUKURI. 


is the part of the medullary substance (p, p) which is 
continued posteriorly beyond the end of the suprarenal, as 
paired cords on the ventral side of the aorta. In fact, the 
part marked p on the left side of fig. 9 is continued from the 
right suprarenal, which has ended several sections in front. 
That this structure is really a part of the medullary sub- 
stance there can be no question, as it is stained brown by 
bichromate of potash. It extends backwards much more 
than the length of the suprarenals themselves. For instance, 
in an embryo of twenty-four days, the right suprarenal 
appears in about forty sections, and the left in about thirty, 
while these cords extend posteriorly for about fifty-five sec- 
tions from the termination of the left suprarenal. The cords 
unite with each other several times, soon, however, sepa- 
rating again. Posteriorly, they gradually lose their brown 
colour, and seem then made up of nerve fibres. Nerves may 
also join them, where they still exhibit the typical brown 
reaction. It is probable, from the comparison of the sec- 
tions of embryos hardened in picric acid and potassium 
bichromate, that so long as the histological structure shown 
in p, fig. 9, is observable, the cords are stained brown. I 
have carefully traced the cords forward both in the longitu- 
dinal and transverse sections, and find that they become con- 
tinuous with the nervous mass (7) shown in fig.8. This is 
obvious from the series of sections in fig. 10. A is the outer-. 
most, the sections gradually going tewards the median axis 
of the body of the embryo. In A the cortex (c) is quite 
large, while the part marked p (same as p, fig. 9) is con- 
tinuous with the medullary substance (m). The nervous 
mass (2, of fig. 8) is here also marked n. In B the cortex 
(c) is disappearing ; in C it has completely disappeared, and 
the part marked p has divided into two parts (p and a). 
Lastly, in D, we see that the nervous mass (mz) has united 
with the small mass marked ain C. Thus there can be 
no doubt that the structure p, figs. 9 and 10, is continuous 
with the nervous mass ”, fig. 8, and must be of a nervous 
nature. And yet it has not a single typical ganglion cell, 
and there is a considerable difference between its microscopic 
appearance and that of the nervous mass (m,figs.8,9,and 10). 
It is possible that the cells that are found in p are of the same 
nature as the smaller cells in the nervous mass. The only 
conclusion that I can arrive at is that this part of the peri- 
pheral sympathetic system becomes early distinguished from _ 
the other parts by an enormous development of connective- 
tissue cells, and by a total absence of ganglion cells, and that 
all this is preparatory to its transformation into the medul- 


DEVELOPMENT OF SUPRARENAL BODIES IN MAMMALIA. 87 


lary substance of the suprarenal. The irregular polygonal 
areas, in which the cells are embedded, are not unlike the 
cell groups of the medullary substance, and might be easily 
transformed into them. This, taken in connection with the 
facts that this structure (p, p, figs. 9 and 10) is directly con- 
tinuous with the medullary substance, and seems gradually 
to pass into it, and that it is stained by bichromate of potash 
seems to justify this conclusion. Its posterior extension 
presents no objection to this view, as we have seen that in 
the adult suprarenal the medullary substance is found 
outside the organ in a corresponding region. 

The results at which I have arrived in the preceding pages 
may be summed up as follows: 

(1) The suprarenal bodies in Mammalia are composed 
of two parts—the cortical and the medullary—totally different 
in their origin. 

(2) The cortical substance arises from the mesoblast. 

(3) The medullary substance is derived from the peri- 
pheral part of the sympathetic system, and is at first 
placed outside of the cortical substance, becoming trans- 
ported into the middle of the suprarenal body in the course 
of development. . 

I may also call attention once more to the interesting 
gradation of the structure of the suprarenals in different 
groups of Vertebrata. In Elasmobranchs the two com- 
ponents of the suprarenals are totally independent of each 
other. The sympathetic part is found in successive sympa- 
thetic ganglia, while the mesoblastic part is a median un- 
paired body. In Reptilia the sympathetic part is no longer 
bound up in sympathetic ganglia, but is closely applied 
against the dorsal side of the mesoblastic part, which is 
paired. In Aves the sympathetic part has found its way 
inside of the mesoblastic part, but is as yet irregularly scat- 
tered throughout the organ (Brunn). Finally, in Mam- 
malia the sympathetic part is collected into one mass, and 
occupies an area in the centre of the mesoblastic part, 
although still showing its origin in its development. 

In conclusion, I wish to express my sincerest obligations to 
Mr. Balfour, with whom the idea of the present investiga- 
tion originated, for his uniform kindness in giving me his 
valuable advice, and also for the facilities which he has 
afforded me in his laboratory for pursuing my work. 


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On the Structure and Duvetopmunt of Leprpostaus. 
By F. M. Batroor, LL.D., F.R.S., and W. N. 
PARKER.’ 


Tue authors commence this paper by thanking Professor 
Alexander Agassiz for the material, both embryological and 
adult, on which these researches were made. 

The first section is devoted to the general development. 
In this section an account is given of the structure of the 
ripe ovum, of the segmentation, of the history of the ger- 
minal layers, of the first development of the principal organs, 
and of the external features of the embryo during embryonic 
and larval life. The more important points established in 
this section are— 

(1.) The ovum when laid is invested by a double covering 
formed of (a) a thick inner membrane, the outer zone of 
which is radially striated, and (4) an outer layer made up of 
highly refractive pyriform bodies which are probably meta- 
morphosed follicular epithelial cells. 

(2.) The segmeutation is complete, though very unequal ; 
the lower pole being very slightly divided into segments, and 
its constituent parts subsequently fusing together to form an 
unsegmented mass of yolk, like the yolk-mass of Teleostei. 

(3.) The epiblast is divided into an epidermic and a 
nervous stratum, as in Teleostei. 

(4.) The walls of the brain, of the spinal cord, and of the 
optic vesicles are formed from a solid medullary keel, like 
that found in Teleostei. 

(5.) The lens, the auditory vesicle, and the olfactory pit, 
are wholly developed from the nervous layer of the 
epidermis. © 

(6.) The segmental or archinephric duct is developed, as 
in Teleostei, from a hollow ridge of the somatic mesoblast, 
which becomes constricted off, except in front; thus forming 
a duct with an anterior pore leading into the body cavity. 

The section on the general development: is followed by a 


1 This paper is an abstra¢t of one which will be published in full in the 
‘ Philosophical Transactions of the Royal Society.’ 
8 


90 F. M. BALFOUR AND W. N. PARKER. 


series of sections on the adult anatomy and development of 
various organs. 

The Brain.—The authors give a fuller description of the 
adult brain than previous anatomists. The new features in 
this description are (1) that the parts identified by previous 
anatomists as the olfactory lobes, are really parts of the 
cerebral hemispheres; the true olfactory lobes being small 
prominences at the base of the olfactory nerves; (2) that 
there is attached to the roof of the thalamencephalon a 
peculiar vesicle, which has not hitherto been noticed, but 
which is similar to the vesicle found by Wiedersheim on the 
roof of the thalamencephalon of Protopterus. They further 
show that the cerebrum is divided into a posterior portion, 
with an unpaired ventricle, and an anterior portion in 
which the ventricle is paired. ‘They consider the presence 
of a portion of the cerebrum with an unpaired ventricle to 
be an indication that this part of the brain retain characters 
which are only found in the embryonic brain of other 
groups. They point to the presence of lobi inferiores on the 
infundibulum, of tori semicirculares in the mid-brain, and 
of a large cerebellum as indications of an affinity between 
the brain of Lepidosteus and that of Teleostei. In the 
embryological section full details are given as to the develop- 
ment of the thalamencephalon, the pineal gland, the cerebrum, 
and the olfactory lobes. 

At the end of the section the characters and affinities of 
the Ganoid brain are dealt with at some length; and the 
authors attempt to show that brains of Ganoids are distin- 
guished (1) by the large size of the thalamencephalon, and 
(2) by the cerebrum being divided into an unpaired portion 
behind and a paired portion in front. 

Organs of Special Sense— Olfactory Sacs.—An account is 
given.of the development of the olfactory sacs, in which 
these sacs are shown to originate as invaginations of the 
nervous layer of the epiblast; the communication between 
the sacs and the exterior being effected by the rupture or 
absorption of the superficial epidermic layer of the epiblast. 
The double opening of these sacs in the adult is described as 
arising from the division of the primitive single opening. 
The olfactory nerve arises as an outgrowth of the brain prior 
to the first differentiation of the olfactory bulb as a special 
lobe of the brain. 

Eye.—lIn the adult eye a vascular membrane is described 
bounding the retinal aspect of the vitreous humour. This 
membrane is supplied by an artery piercing the retina close 
to the optic nerve, and the veins from it fall into a circular 


STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 9L 


vessel placed at the insertion of the iris. The membrane 
itself is composed of a hyaline ground substance with 
numerous nuclei. 

In the developmental section devoted to the eye the main 
subject dealt with is the nature of the mesoblastic structures 
entering the cavity of the optic cup through the choroid 
slit. It is shown that a large non-vascular mesoblastic 
process first enters the optic cup, and that together with the 
folded edge of the choroid slit it forms a rudimentary and 
provisional processus falciformis. At a later period an 
artery, bound up in the same sheath as the optic nerve, 
enters the optic cup, and the vascular membrane found in 
the adult then becomes developed. 

The Suctorial Disc——The structure of a peculiar larval 
suctorial organ, placed at the end of the snout, is described, 
and the organ is shown to be formed of papille composed of 
elongated epidermic cells, which are probably glandular 
(modified mucous cells), and pour out a viscid secretion. 

Muscular System.—The lateral muscles of Lepidosteus 
are shown to differ from those of other fishes, except the 
Cyclostomata, in not being divided into a dorso-lateral and 
ventro-lateral group, on each side of the body. 

Vertebral Column and Ribs.—This section of the paper 
commences with a description of the vertebral column and 
ribs of the adult. In this part special attention is called to 
a series of cartilaginous elements, placed immediately below 
the ligamentum longitudinale superius, which appear to have 
escaped the notice of the anatomists who have previously 
worked at Lepidosteus. These elements are shown to be 
intervertebrally situated. 

With reference to the ribs the authors point out that for 
the greater part of their length they course along the bases 
of the intermuscular septa, immediately external to the 
peritoneal membrane, but that their free extremities bend 
outwards and penetrate between the muscles along the 
intermuscular septa till they nearly reach the skin. 

In the embryological part of this section a detailed account 
is given of the development of the vertebral column, of which 
the following is a summary : 

There is early formed round the notochord a meso- 
blastic investment which is produced into two dorsal and 
two ventral ridges, the former uniting above the spinal 
cord. Around the cuticular sheath of the notochord an 
elastic membrane, the membrana elastica externa, is next 
developed. The neural ridges become enlarged at each 
intermuscular septum, and these enlargements soon become 


92 F. M. BALFOUR AND W. N. PARKER. 


converted into cartilage, thus forming a series of neural 
processes, riding on the membrana elastica externa, and 
extending about two thirds of the way up the sides of the 
spinal cord. Hzmal processes arise simultaneously with 
and in the same manner as the neural; they are small in 
the trunk, but at the front end of the anal fin they suddenly 
enlarge and extend ventralwards. Behind this point each 
succeeding pair of hemal processes becomes larger than the 
one in front, each process finally meeting its fellow below 
the caudal vein, thus forming a completely closed hemal 
arch. These arches are, moreover, produced into long 
spines supporting the fin-rays of the caudal fin, which thus 
differs from the other unpaired fins in being supported by 
parts of the vertebral column, and not by separately formed 
skeletal elements. 

In the next stage which the authors have had the oppor- 
tunity of studying (a larva of 54 centims.), a series of well- 
marked vertebral constrictions are to be seen in the noto- 
chord. The sheath is now much thicker in the vertebral 
than in the intervertebral regions ; this being due to a special 
differentiation of a superficial part of the sheath, which 


appears more granular than the remainder, and forms a 


cylinder in each vertebral region. Between it and the 
gelatinous tissue of the notochord there remains a thin 
unmodified portion of the sheath, which is continuous with 
the intervertebral parts of the sheath. The neural and 
hemal arches, which are of course placed in the vertebral 
regions, are now continuous with a cartilaginous tube embrac- 
ing the intervertebral regions of the notochord, and con- 
tinuous from one vertebra to the next. A delicate layer of 
bone, developed in the perichondium, invests the cartila- 
ginous neural arches, and this bone grows upwards so as 
to unite above with the osseous investment of separately 
developed bars of cartilage, which are directed obliquely 
backwards. These bars, or dorsal processes, may be reckoned 
as parts of the neural arches. Between the dorsal processes 
of the two sides are placed median rods of cartilage, which 
are developed separately from the true neural arches, and 
which constitute the median spinous elements of the adult. 
Immediately below these rods is placed the ligamentum 
longitudinale superius. There is now the commencement, 
not only in the tail but also in the trunk, of a separation 
between the dorsal and ventral parts of the hemal arches 
where the latter pass ventralwards, on each side of the body 
cavity, along the lines of insertion of the intermuscular septa, 
They are obviously the ribs of the adult, and there is no 


— = 


STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 93 


break of continuity of structure between the hemal arches 
of the tail and the ribs. In the anterior part of the trunk, 
the ribs pass outwards along the intermuscular septa till 
they reach the epidermis. Thus the ribs are originally 
continuous with the hzmal processes. Behind the region 
of the ventral caudal fin the two hzmal processes merge into 
‘ one, which is not perforated by a canal. 

Each of the intervertebral rings of cartilage becomes 
eventually divided into two parts, which are converted into 
the adjacent faces of contiguous vertebra, the curved line 
where this will be effected being plainly marked out at a 
very early stage. As these rings are formed originally by 
the spreading of the cartilage from the primitive neural and 
hzemal processes, the intervertebral cartilages are clearly 
derived from the neural and hemal arches. The inter- 
vertebral cartilages are thicker in the middle line than at 
their two ends. 

In the latest stage examined (11 centims. long) the verte- 
bral constrictions of the notochord are rendered much less 
conspicuous by the intervertebral cartilages giving rise to 
marked intervertebral constrictions. In the intervertebral 
regions the membrana elastica externa has become aborted 
at the posterior border of each vertebra, and the remaining 
part is considerably puckered transversely. The inner sheath 
of the notochord is puckered longitudinally in the inter- 
vertebral regions. The granular external layer of the sheath 
in the vertebral regions is less thick than in the last stage, 
and exhibits a faint radial striation. 

Two closely approximated cartilaginous elements now form 
a key-stone at each neural arch above; these are directly 
differentiated from the ligamentum longitudinale superius, 
into which they merge above. An osseous plate is formed 
on the outer side of each of these cartilages. These plates 
are continuous with the lateral osseous bars of the neural 
arches, and give rise to the osseous part of the roof of the 
spinal canal of the adult. Thus the greater part of the 
neural arches is formed by membrane bone. 

The hemal arches are invested by a thick layer of bone, 
and there is also a continuous osseous investment round the 
vertebral portions of the notochord. The intervertebral 
cartilages become penetrated by branched processes of 
bone. 

The embryological part of this section is followed by a 
comparative part treated under three headings. In the first 
of these the vertebral column of Lepidosteus is compared 
with that of other forms; and it is pointed out that there 


94. F. M. BALFOUR AND W. N. PARKER. 


are grave difficulties in the way of comparing the vertebre 
of Lepidosteus with those of the Urodela, in the fact that in 
Lepidosteus the intervertebral cartilages originate from the 
bases of the arches, while in the Urodela they are stated by 
Gotte to be thickenings of a special cartilaginous investment 
of the notochord, which would seem to be homologous with 
that cartilaginous sheath which is placed in Elasmobranchii 
and Dipnoi within the membrana elastica externa. On the 
other hand, the development of the vertebrz of Lepidosteus 
is shown to resemble in most features that of Teleostei, from 
which it mainly differs in the presence of intervertebral 
cartilaginous rings. 

In the second section, devoted to the homologies of the 
ribs of Pisces, the conclusions arrived at are summed up as 
follows : 

The results of the authors’ researches appear to leave two 
alternatives as to the ribs of fishes. One of these, which 
may be called Gottes view, may be thus stated:—The 
hemal arches are homologous throughout the Pisces; in 
Teleostei, Ganoidei and Dipnoi, the ribs, placed on the inner 
face of the body wall, are serially homologous with the 
ventral parts of the hemal arches of the tail; in Elasmo- 
branchii, on the other hand, the ribs are neither serially 
homologous with the hemal arches of the tail, nor homo- 
logous with the ribs of Teleostei and Ganoidei, but are out- 
growths of the hemal processes into the space between the 
dorso-lateral and ventro-lateral muscles, and outgrowths 
which may perhaps have their homologies in Teleostei and 
Ganoids in certain accessory processes of the vertebre. 

The other view, which the authors are inclined to adopt, 
is as follows :—The Teleostei, Ganoidei, Dipnoi and Elasmo- 
branchii are provided with homologous hzmal arches, which 
are formed by the coalescence below the caudal vein of 
simple prolongations of the primitive hemal processes of the 
embryo. The canal enclosed by the hemal arches can be 
demonstrated embryologically to be the aborted body cavity. 

In the region of the trunk the hzmal processes and their 
prolongations behave somewhat differently in the different 
types. In Ganoids and Dipnoi, in which the most primitive 
arrangement is probably retained, the ribs are attached to 
the hzmal processes, and are placed immediately without the 
peritoneal membrane, at the insertion of the intermuscular 
septa. These ribs are in many instances (Lepidosteus, 
Acipenser), and very probably in all, developed continuously 
with the hemal processes, and become subsequently seg- 
mented from them. They are serially homologous with the 


SS, a eee 


STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 95 


ventral parts of the hemal arches of the tail, which, like 
them, are in many instances (Ceratodus, Lepidosteus, Poly- 
pterus, and to some extent in Amia) segmented off from the 
basal parts of the hzmal arches. 

In Teleostei the ribs have the same position and relations 
as those in Ganoids and Dipnoi, but their serial homology 
with the ventral parts of the hemal processes of the tail is 
often (e.g. the Salmon) obscured by the anterior hemal 
arches (7.e. those in the posterior part of the trunk) being 
completed, not by the ribs, but by independent outgrowths 
of the basal parts of the hemal processes. 

In Elasmobranchii a still further divergence from the 
primitive arrangement is present. The ribs appear to have 
passed outwards, along the intermuscular septa, into the 
muscles; and are placed between the dorso-lateral and 
ventro-lateral muscles (a change of position of the ribs of the 
_ same nature is observable in Lepidosteus). This change of 
position, combined probably with the secondary formation 
of a certain number of anterior hzmal arches, similar to 
those in the Salmon, renders their serial homology with the 
ventral parts of the hemal processes of the tail far less clear 
than in other types; and further proof is required before 
such homology can be considered as definitely established. 

Under the third heading the skeletal elements supporting 
the fin-rays of the ventral lobe of the caudal fin of various 
types of fishes are compared, and the following conclusions 
are arrived at: 

(1.) The ventral lobe of the tail-fin of Pisces differs from 
the other unpaired fins in the fact that its fin-rays are 
directly supported by spinous processes of certain of the 
hzmal arches, instead of by indently developed interspinous 
bones. 

(2.) The presence or absence in the tail-fin of fin-rays, 
supported by hzmal arches, may be used in deciding 
whether apparently diphycercal tail-fins are aborted or 
primitive. 

Urogenital Organs.— With reference to the character of the 
adult urogenital organs, the authors show that for the female 
the descriptions of Miiller and Hyrtl are substantially 
accurate, but that Hyrtl’s description of the generative ducts 
of the male is wholly incorrect. 

They find that in the male the semen is transported from 
the testes by means of a series (40—50) of vasa efferentia, 
supported by the mesorchium. In the neighbourhood of the 
kidney these vasa unite into a longitudinal canal, from which 
transverse trunks are given off, which become continuous 


96 F. M. BALFOUR AND W. N. PARKER. 


with the uriniferous tubuli. The semen is thus transported 
through the kidney into the kidney-duct (segmental duct), 
and so to the exterior. No trace of a duct homologous with 
the oviduct of the female was found in the male. 

With reference to the development of the excretory system, 
the authors have established the following points : 

(1.) That the segmental (archinephric) duct is developed 
as in Teleostei. 

(2.) That a pronephros, resembling in the main that of 
Teleostei, is developed from the anterior end of the segmental 
duct. But they find that the pronephric chambers, each 
containing a glomerulus, into which the coiled pronephric 
tubes open, are not, as in Teleostei, completely shut off from 
the body cavity, but remain in communication with it by 
two richly ciliated canals, one on each side of the body. 

(3.) The pronephros eventually undergoes atrophy. 

(4.) Some of the mesonephric tubes have peritoneal 
funnels in the larva. 

(5.) The ovarian sac continuous with the oviduct is estab- 
lished by a fold of the peritoneal membrane, near the attach- 
ment of the mesovarium, uniting with the free edge of the 
ovarian ridge to form a canal, the inner wall of which is 
constituted by the ovarian ridge itself. 

(6.) The posterior part of the oviduct is not formed until 
the ovarian sac has become developed, and had not been 
developed in the oldest larva (11 centims.) the authors have 
succeeded in obtaining. 

The Alimentary Canal and its Appendages.—In this section 
the authors give a detailed account of the topographical 
anatomy of the alimentary tract in the adult. They have 
detected a small pancreas close to the bile-duct, and call 
special attention to a ventral mesentery passing from the 
posterior straight section of the intestine to the ventral wall 

_ of the body. 

In the embryological part of the section a detailed account 
is given of the development (1) of the pancreas, which is 
described as arising as a dorsal diverticulum of the duodenum 
on a level with the opening of the bile-duct ; (2) of the yolk 
sac and vitelline duct; (8) of the spiral valve, which first 
appears as a hollow fold in the wall of the intestine, taking 
a slightly spiral Course, and eventually becoming converted 
into a simple spiral ridge. 

The so-called hyoid gill, which the authors expected to 
find well developed in the larva, is shown not to be found 
even in the oldest larva the head of which was examined (26 
mm.). 


eee Sa 


STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 97 


The last section of the paper is devoted to the considera- 
tion of the systematic position of lLepidosteus. The 
Teleostean affinities of Lepidosteus are brought into promi- 
nence, but it is shown that Lepidosteus is nevertheless a true 
Ganoid. 

The arguments used in this portion of the paper do not 
admit of being summarised. 


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On Certain Points in the Anatomy of Curton. By 
Avam Szpewick, M.A., Fellow of Trinity College, 
Cambridge. 


An account of the structure of the kidney of Chiton has 
long been a want in morphology. Middendorff,! in 1848, 
described a branched gland lying ventrally on each side of the 
body cavity which he identified as kidney ; but he records 
no observation on the structure of the gland, and expressly 
states that he was not able to make out its opening or 
relation to other organs. Schiff,” ten years later, was unable 
to find this gland in Chiton piceus, and throws doubt on 
Middendorff’s interpretation of its function. 

Von Jhering® has comparatively recently recorded some 
observations on the kidney of Chitou, and starts from the 
position that no kidney is known in Chiton, Middendorff’s 
view as to the nature of the branched gland having been 
sufficiently refuted by Schiff’s later observations. Von 
Jhering states that in the species of Chiton observed by 
him the kidney consists of a branched gland lying ventral to 
the rectum in the hinder part of the body cavity, and that 
it opens by a single median pore ventral to the anus. He 
further figures this opening. 

While staying at Herm this summer I found a fair 
number of a good-sized species of Chiton—Chiton discrepans ; 
and the results which I have obtained from the study of the 
anatomy of this form, especially those which concern the 
kidney, seem to me sufficiently important for immediate 
publication. In the first place, I may mention that I have 
seen nothing in any of my dissections or sections which in 
the least supports von Jhering’s statements as to the exist- 
ence of a median renal duct and opening; and that my 
observations are entirely opposed to the conclusion arrived 
at by this investigator as to the unpaired nature of the 
kidney of Chiton. On the contrary, Middendorff’s observa- 

1 «Mémoires de |’Acad. de St. Pétersbourg,’ 6th ser., vol. vi. 


2 Schiff, ‘Zeit. f. Wiss. Zool.,’ Bd. ix. 
3 Morphol. Jahrbuch,’ Bad. iv, 


100 ADAM SEDGWICK. 


tions, so far as they went, were perfectly correct. The 
paired lateral-branched gland described by the latter observer 
is part of the kidney. 

The kidney of Chiton is a paired gland with paired open- 
ings into the pallial groove and into the pericardium, and is 
constructed on the type always found in molluscan renal 
organs (fig, 1). It opens in the species I have chiefly 
examined (Chiton discrepans) into the pallial groove (fig. 
1, 7.0.) internal to, but on a level with the last gill (16). 
The duct ruus from the opening round the outside border of 
the pallial nerve (fig. 2, 7.0.), and then passes inwards to 
open into a bladder-like structure placed in the hody cavity 
(fig. ], D, and fig. 2, D). This bladder-like structure lies 
close to the body wall immediately beneath the pericardium 
(fig. 2, p.c.), and it does not seem to extend backwards 
beyond the last gill. 

On a closer examination by means of sections, it is seen 
to be beset by a number of branched glandular ceca, lying 
in the hinder part of the body cavity (fig. 2, k.t.), which 
open into it, and into a backward prolongation from it 
(fig. 1, 4.4.). These branched glandular ceca on opening 
the body cavity are seen as a mass of tubes apparently inter- 
lacing with those of the opposite side, and lying ventral to 
the rectum on the floor of the body cavity (fig. 2, k.¢.). This 
portion of the kidney has been seen and described by von 
Jhering, but instead of constituting the whole of the kidney 
and opening to the exterior by a median pore, it is only the 
posterior part, and opens on each side into the bladder-like 
structure which opens to the exterior in the position 
described above. I have many series of sections through 
this hinder part of the kidney of Chiton, which prove most 
conclusively that these hinder ventrally-placed tubules do 
open in the way I have stated. 

On examining the anterior end of the bladder-like structure 
it is found that it is continued forwards as a duct (fig. 1, £.d.), 
which receives, all along its course, the ducts of bunches of 
branching glandular ceca, lying at the side of the body © 
cavity (fig. 1, k.¢.). These branching glandular cxca con- 
stitute the gland described by Middendorff. Their structure 
precisely resembles that of the first described posterior 
tubules, which open into the dilated part of the duct and its 
backward prolongation. The duct can be traced forward to 
about the level of the fourth shell-plate (fig. 1, T), at which 
point it turns sharply round and runs back parallel with the 
first part of its course. A considerable part of the gland 
lies in front of this turning point of the duct ; the secretion 


CERTAIN POINTS IN THE ANATOMY OF CHITON. 101 
Pie. ols 


A diagrammatic representation of the kidney and generative ducts of 
Chiton discrepans, viewed from the ventral surface. The pallial groove is 
represented as enclosed by the lines p.g.; and in it are seen the 16 gills 
(or.), the generative (g.o.) and renal (r.0.) orifices; andthe anus (@.). The 
branched nature of the kidney is shown in the anterior part of the figure on 
the right side; posteriorly these secreting tubules are omitted. On the left 
side of the figure the kidney duct alone is indicated. 

a., anus; 6r., branchie ; D, dilated part of kidney duct opening to ex- 
terior ; g. points to the junction of the generative duct with the generative 
gland; the generative gland is supposed to be torn away; g.d., generative 
duct; 4.4., posterior part of kidney duct; g.o., generative orifice; 4.t., 
secreting tubules of kidney ; 4.d., duct of kidney running forward, bending 
round at T and running back, receiving glands as far back asO. From O 
it runs to the pericardial opening p.o., receiving no glands; p.g., pallial 
groove; 13, 14, 15, 16, last four branchie; tle ventricle and auricular 
openings are indicated by dotted lines, 


102 ADAM SEDGWICK. 


of this part is poured into a branch given off from the main 
duct atthe bend (fig: 1, T). - The posteriorly directed part 
of the renal duct lies close to the dorsal edge of the part 
running forward, and, like the latter, receives the efferent 
ducts of bunches of glandular ceca (fig. 1). From the level 
of the fifth shell-plate (fig. 1, O) to its posterior termination 
(fig. 1, p.o.), about to be described, it receives no glandular 


Fie, 2. 


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A diagrammatic representation of a transverse section through Chiton 
discrepans at the level of the renal orifices (r.0., fig. 1). Dorsally is the 
pericardial cavity with the heart, separated by the pericardial floor from the 
general body cavity (4.c.), containing the viscera. Ventrally is the poste- 
rior apparently median unpaired part of the kidney (k.¢. and &.c.) seen by 


von Jhering. A little in front of this section the kidney tubules take up a 
distinctly lateral position. 


Dj py.5 62.3 41.5 f.0. asic, 1. 

A, auricle; V, ventricle; 4.0., branchial vein; 4.a., branchial artery ; 
p.c., pericardial cavity; /.z., lateral nerve (pallial); p.z., pedal nerve; 
F, foot; A.C., alimentary canal; y.g., generative gland; d.c., body cavity ; 


kc. see h.t.; p.k.d., part of kidney duct which in fig. 1 is hidden from view 
by D. 


ceca, but runs backwards as a simple duct distinguishable 
by its brown colour, which is due to a deposit of colourin 

matter in its walls. On reaching the level of the bladder- 
like dilatation of the kidney duct first described, it applies 
itself to the dorsal inner wall of that structure as far back 
as the level of the last gill. At this point, which marks the 


CERTAIN POINTS IN THE ANATOMY OF CHITON. 103 


hind border and the external opening of the bladder, it runs 
outwards and then forwards (fig. 2, p.k.d.) in close contact 
with the dorsal side of the lateral nerve cord. It runs forward 
to about the level of the penultimate gill, where it suddenly 
stops and opens by a small pore into the pericardium (fig. 
1, p.o.) beneath, z.e. ventral to the anterior part of the 
auricle. 

Comparing the arrangement of the kidney of Chiton with 
that of Anodon, there is seen to be a close agreement. In 
both the kidney is paired and consists of a gland bent on 
itself, opening at the one extremity into the pallial cavity, 
and at the other into the pericardium. In both the kidney 
is unsegmented (a fact to be remembered when the nature 
of the shell and gills of Chiton is discussed). There is a 
further agreement between these two animals in the 
relation of the openings of the generative ducts to those 
of the renal ducts; in both the latter are placed close 
behind the former. 

With regard to the minute structure of the kidney of 
Chiton I have no exact observations. It is necessary to 
study it in the fresh state. The inner borders of the cells 
lining the glandular ceca are stated by von Jhering to be 
ciliated. 

The most internal part of the kidney duct, 7.e. that which 
receives no glandular ceca (fig. 1, O to p.k.d.), is, with the 
exception of a small portion adjoining the pericardial open- 
ing, lined by columnar cells containing a yellow colouring 
matter, which gives this portion of the duct a yellow colour, 
easily visible to the naked eye. This yellow colouring 
matter, which seems to be part of the excretion of the cells 
lining the duct, is absent in the part of the duct which runs 
forwards from the level of the hinder edge of the bladder to 
the pericardial opening (p.k.d. to p.o.). Here are found 
large columnar cells provided with long cilia, which line also 
the pericardial opening. 

The cells of the glandular ceca seem to have the struc- 
ture usually seen in molluscan renal organs, and have been 
correctly described by von Jhering. 

To sum up, the kidney of Chiton consists of— 

(1.) A duct opening to the exterior in the pallial groove 
behind the generative opening, and internally into the 

ericardium. 

(2.) Glandular czeca opening into this duct. 

The duct may be described as consisting of three parts : 

(1.) The part into which the glandular ceca of the kidney 
open. This part is open behind where it opens to the 


104 ADAM SEDGWICK. 


exterior (fig.1, D). In front it bends round (fig. 1, T), and 
runs backward to about the level of the fifth shell plate, 
where it changes its character, and is continuous with (2) a 
duct (fig. 1, O) containing brown colouring matter in the 
columnar cells lining it, and receiving no glandular ceca. 
This part extends back to the level of the last gill, where it 
turns outwards, and becomes continuous with (3) a part 
running forward for a short distance close to the lateral 
nerve, and lined by large ciliated columnar cells. This part 
opens in front at the level of the penultimate gill into the 
pericardium (fig. 1, p.o.). I expected to find the communi- 
cation between the two parts of the renal duct behind in the 
region of the bladder, and for some time I was puzzled at 
not finding it. On mentioning the arrangement of parts 
to Mr. Balfour, he suggested that the communication might 
possibly be found in front, reasoning from the analogy of 
the structure of the kidney in other Mollusca. On examin- 
ing the anterior part of the gland more carefully, I at once 
found that his suggestion was correct, the two parts of the 
gland communicating as I havedescribed. I have no obser- 
vations to add to those of previous observers, on the general 
arrangement of the nervous system. I may mention that 
the lateral and pedal nerves have a coating of ganglion cells 
and a central core of fibres. 

The animals are diecious. The generative gland is 
unpaired and dorsal. The generative ducts are paired, and 
are attached to the hinder border of the gland, and open in 
Chiton discrepans into the pallial groove between the thir- 
teenth and fourteenth gill, in a line with the opening of the 
renal duct. The duct passes dorsal to the anterior end of 
the dilated part of the renal duct (fig. 1, g.d.), and then 
curls round the outer border of the lateral nerve cord to 
its opening, presenting in this respect precisely the same 
relation as does the renal duct. The male duct has a short 
direct course to its opening (fig. 1), while the female duct 
is much coiled. 

Another species, Chiton cancellatus, which I have examined, 
presents essentially the same arrangement of its renal 
organ and generative ducts as that just described for Chiton 
discrepans. 

Dall! states that in some species of Chiton the generative 
products escape into the body cavity and make their exit by 
several pores placed close together, and symmetrically, on 
each side in the pallial groove; oviducts apparently being 


1 «Proceedings of the United States’ National Museun,’ vol. i. 


CERTAIN POINTS IN THE ANATOMY OF CHITON. 105 


absent. I have not any specimens of the species he mentions 
as possessing this peculiarity (e.g. Chiton marmoreus and ruber), 
so have not been able to test his observations by means of 
sections. 


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On the Germinat Layers and Harty Devetopment of 
the Motz. By Watrer Heaps. 


Tue following is a note on some investigations which I 
have been carrying on by the kindness and with the help of 
Mr. Balfour, in the Morphological Laboratory, Cambridge, 
upon the origin and formation of the germinal layers in 
mammals, more especially in the mole (Talpa Europea). I 
hope shortly to be able to give a more complete account. 

In the communication the following subjects are dealt 
with : 

(1.) The origin of the epiblast. 

(2.) The mode of development of the mesoblast. 

(3.) The structure of the neurenteric canal. 

(4.) The relations of the mesoblast and the hypoblast to 
the notochord. 

Recent investigations have left the earlier phases of mam- 
malian development in some confusion, it may therefore be 
advisable briefly to mention the more important views which 
are entertained on this subject. 

Professor Edward van Beneden, in a paper entitled “ La 
formation des feuillets chez de Lapin’”’ (‘Archives de 
Biologie,’ vol. i, Part 1, 1880), gives an account of the 
segmentation of the ovum of that animal, and states that 
during segmentation a differentiation of the segmentation 
spheres into two layers is established, the one of which 
grows over and encloses the other, giving rise in this manner 
to what Van Beneden calls a metagastrula. The outer of 
these two layers he terms ectoderm and the inner entoderm, 
names which seem to me, for reasons which will appear in 
the sequel, to be misleading, and for which I propose to sub- 
stitute the terms outer and inner layers respectively. 

Subsequently, according to Van Beneden, a cavity, the 
blastodermic cavity, is developed between the outer and inner 
layers of cells; the cells of the former layer become flattened 
and multiply, and form the wall of the so-called blastodermic 
vesicle ; at the same time the blastodermic cavity is enlarged, 


108 WALTER HEAPE. 


while the inner layer remains as a rounded mass of cells 
attached to the wall of the vesicle over a small area known 
as the embryonic area. Van Beneden considers that the 
outer layer of cells forms the permanent epiblast, both of 
the embryonic area and of the blastodermic vesicle, while 
the inner mass of cells breaks up into two layers, a lower 
single layer of flattened cells, the hypoblast, and a layer of 
cells which he calls the mesoblast, lymg between the hypo- 
blast and the epiblast of the embryonic area. 

Professor Kolliker, on the other hand, writing in the 
‘Zoologischer Anzeiger’ (Nos. 61 and 62, vol. iii, 1880), 
“Die Entwicklung der Keimblatter des Kaninchens,” does 
not dispute the presence of Van Beneden’s epiblast, hypo- 
blast, and mesoblast, in the stage of development described 
above, but states his agreement with an earlier view of 
Rauber, that in the region of the embryo the outer of these 
layers disappears, while the whole of the middle layer 
becomes converted into the epiblast of the embryonic area ; 
the epiblast of the remainder of the vesicle, however, he 
considers is formed from part of the original outer layer of 
cells. The mesoblast owes its origin, in his opinion, wholly 
to a budding from the epiblast of the primitive streak. 

Professor Lieberkiihn published in Marburg, in 1879, in a 
paper “‘ Ueber die Keimblatter der Saugethiere,” the results 
of his researches upon the dog and mole, in which he states 
that the epiblast of the embryonic area is derived from the 
greater part of the primitive inner mass of cells (that portion 
in fact forming Van Beneden’s mesoblast), together with 
the part of the original outer layer of cells which overlies 
the inner mass; the hypoblast he derives from the inner 
mass of cells, while the mesoblast he believes to be formed 
from both epiblast and hypoblast in the region of the 
primitive streak. 

I myself have been fortunate enough to secure a fairly 
complete series of mole embryos ranging from an early 
appearance of the blastodermic cavity until the formation of 
the medullary groove; an examination of which leads me, in 
the main, to agree with Lieberktihn’s account of the develop- 
ment of the embryonic layers of that animal. 

I have not been able to follow completely the course of 
segmentation, nor have I been able to trace a differentiation 
of the segments into two layers, an outer and an inner, 
though there appears to be no doubt, on account of the 
arrangement of the spheres in a somewhat later stage, that 
Van Beneden’s description of a fully segmented ovum is 
substantially correct, 


GERMINAL LAYERS AND EARLY DEVELOPMENT OF MOLE, 109 


The earliest specimen of an ovum in my possession after 
the completion of segmentation is similar to that figured in 
Van Beneden’s paper (loc. cit.), Plate iv, fig. 6, ii, and in 
Lieberkiihn’s paper (loc. cit.), fig. 1. | 

The ovum consists of an outer layer and an inner mass of 
cells, between and partly separating which is a cavity. The 
outer layer has the form of a sphere of somewhat flattened 
cells, while the inner mass is composed of irregularly polygonal 
cells ; these two are attached together for a small area, else- 
where they are separated by a cavity, the blastocermice cavity, 
which is seen in optical section as a crescent-shaped space 
partially surrounding the inner mass of cells. The diameter 
of this ovum measures ‘1] mm., and that of the inner mass 
of cells ‘06 mm. A thick zona invests the ovum. 

Upon the formation of the blastodermic cavity the ovum 
may be called the blastodermic vesicle. 

The vesicle becomes enlarged, and I am inclined to 
believe that during the enlargement the cells of the inner 
mass assist in the formation of the outer wall of the vesicle, 
since in various vesicles of about ‘2, ‘25, °3, ‘38, mm. diam- 
eter, the diameter of the inner mass, which is of an approx- 
imately spherical shape, is less, being respectively *04, -04, 
"04, 05 mm., than in the youngest vesicle, measuring as 
stated above, ‘11 mm. in which the inner mass is ‘06 mm. 
diameter. 

Sections through a vesicle measuring ‘25 mm. diameter 
(the inner mass measuring about ‘04 mm.) show the outer 
layer to be composed of greatly flattened cells closely 
applied to the zona, which is now much thinner, owing to 
the expansion of the vesicle, while the inner mass in the 
form of a solid mass of irregularly rounded cells is attached 
to the outer layer for a small circular region, which I shall 
speak of as the embryonic area. 

As the vesicle enlarges the inner mass of cells slightly 
flattens out and widens at the same time, so that the embry- 
onic area becomes enlarged. 

In a vesicle ‘44 mm. diameter the inner mass of cells is 
seen to be commencing to divide into two layers, and ina 
vesicle ‘57 mm. in diameter, in which the inner mass is ‘08 
mm. in diameter, this division is completed, and a layer 
composed of a single row of slightly flattened cells is sepa- 
rated off from and underlies the main portion of the inner 
mass of cells; this layer is the hypoblast. The main 
portion of the inner mass of cells is undergbing at the same 
time a change in structure, inasmuch as some of the polyg- 


110 WALTER HEAPE. 


onal or rounded cells of which it has hitherto been composed 
now become elongated and columnar, 

The hypoblast in an oval blastodermic vesicle of about ‘88 
mm. by ‘81 mm., is still formed of slightly flattened cells 
beneath the embryonic area, but it has grown and extended 
beyond that area, so that its outer part lies beneath and in 
close contact with the outer layer of the blastodermic vesicle ; 
the cells of this portion of the hypoblast are wide and much 
fiattened, and their nuclei stain deeply with hematoxylin. 

A cavity appears about this stage of development in the 
region of the embryonic area between the flattened outer 
layer and the inner mass, the cells of the latter having now 
largely become columnar. In the vesicle last mentioned 
(‘88 mm. by *81 mm.), nearly the whole of the inner mass 
has become transformed from a rounded mass of polygonal 
cells into a concave plate of columnar cells, forming the 
floor of a cavity which is roofed over by the cells of the outer 
layer of the blastodermic vesicle. In this cavity a few cells 
are placed, which are connected with the outer layer or inner 
mass, or with both of these, by means of protoplasmic pro- 
cesses; I believe these cells to be cells of the inner mass 
which have not yet become columnar. 

Lieberkthn states that some of the cells of the inner mass 
grow round and above the cavity just described, which thus 
comes to lie within the inner mass. The specimens from 
which he derives his opinion, however, were, I believe, pre- 
served in Miiller’s fluid. I have myself seen a similar 
apparent arrangement in such preparations, which upon 
comparison with sections of vesicles of similar ages prepared 
in picric acid appear to me to bear a different interpretation, 
the layer of cells above the cavity being formed of the flat 
outer layer cells with a few more or less isolated cells of 
the inner mass. 

In a vesicle of about ‘97 mm. diameter the inner mass of 
cells has still the form of a concave plate composed of two or 
three layers of for the most part columnar cells ; the flattened 
cells of the outer layer remain, as in the previously described 
specimen, closely attached to the zona, and the cells lying 
in the cavity are fewer, while some of them appear to have 
been drawn on to the concave plate and transformed into 
columnar cells. Cells in a transition stage may be seen on 
the surface of the plate. 

At a later stage the concave plate extends itself, the 
curvature becoming less, and eventually approaches to and 
finally comes into contact with the flat cells forming that 
portion of the wall of the vesicle which in the previously 


———_ OE ee —— 


GERMINAL LAYERS AND EARLY DEVELOPMENT OF MOLE. I11 


described specimens lay above the plate; the flattened cells 
of this part soon become columnar and the fusion between 
them and the plate becomes complete. 

Somewhat prior to this stage the edges of the plate 
become continuous with the outer layer of the wall of the 
vesicle beyond the region of the embryonic area. 

Thus, the greater part of the inner mass of cells, as Lieber- 
kihn correctly states, combines in the form of a plate of more 
or less columnar cells with that part of the flat outer layer of 
cells which immediately overlies it, to form a plate of columnar 
cells two and three rows deep ; this plate is the epiblast plate 
of the embryonic area, the remainder of the outer layer of 
cells forming the epiblastic wall of the blastodermic vesicle. 

The portion of the inner mass of cells which was separated 

off from the main mass as the hypoblast still forms only a 
single row of somewhat rounded cells, the central part of 
which underlies the embryonic area, while the peripheral 
part continually extends as a layer of flattened cells along 
the inner aspect of the epiblastic wall of the remainder of the 
blastodermic vesicle. 
- In concluding this portion of my subject I may add, in 
support of what I believe in harmony with Lieberkthn to be 
the origin of the true epiblast of the embryonic area in the 
mole, that in the course of my work on this subject, carried 
on since the investigations of Mr. Balfour and myself, 
published in the second volume of Mr. Balfour’s ‘ Com- 
parative Embryology,’ I have obtained from an embryo 
rabbit of six days four hours old, sections which appear to 
me conclusively to confirm the results at which we before 
arrived, namely, that the epiblast plate of. the embryonic 
area is derived (as in the mole) conjointly from the at first 
flattened cells of the primitive outer layer (called by Rauber 
and Kolliker ‘‘ Deckzellen,” and stated by those observers to 
disappear from the embryonic area), and from the larger 
portion of the primitive inner mass of cells (held by Van 
Beneden to be the true mesoblast, and stated by Kolliker 
alone to form the epiblast plate. In the sections of this 
embryo the cells of the already described primitive outer 
layer are seen in a transition stage, being wedge-shaped and 
prolonged in between the cells of the inner mass. 

At the stage of growth now arrived at the blastodermic 
vesicle may be considered to consist of an embryonic and a 
non-embryonic portion. A surface view shows the embryonic 
area to be in the form of a more or less circular opaque disk. 
The wall of the vesicle consists of a two-layered and a single- 
layered portion ; the latter, formed of epiblast, alone com- 


113 WALTER HEAPE, 


prises the portion of the wall opposite to the embryonic area, 
while the former, consisting of epiblast and hypoblast, forms 
the embryonic area and the part of the vesicle immediately 
adjoining it. 

In the course of further growth the vesicle greatly enlarges, 
and the zona becomes much attenuated, affording but little 
support to the now also exceedingly thin and delicate wall of 
the vesicle ; it therefore becomes difficult to obtain specimens 
in good preservation. 

In the earliest specimen of this stage which I possess the 
embryonic area is oval, measuring ‘74 by ‘48 mm. In a 
surface view a dark line or band is seen to run along the centre 
of the hinder third of the area. This is the well-known primi- 
tive streak; it is narrow anteriorly, while posteriorly it 
becomes broader, and finally behind takes up nearly the 
whole breadth of the embryonic area; it is due to the 
presence of a third layer of cells, the mesoblast, between the 
epiblast and hypoblast. Transverse sections of this em- 
bryonic area show the major portion in front of the primitive 
streak to be composed (1) of a plate of epiblast formed of 
two or three rows of columnar cells, and (2) of a single layer 
of rounded hypoblast cells somewhat flattened towards the 
edge of the area. Immediately in front of the primitive 
streak there appears, extending entirely across the area, a 
layer of mesoblast, which is not connected with the epiblast, 
but is so intimately united with the hypoblast in the middle 
line, that the two layers cannot there be clearly distinguished, 
though towards the periphery of the area they are quite 
distinct. A section taken through the anterior end of the 
primitive streak discovers a narrow band of epiblast cells in 
the middle line, giving rise by budding to a layer of mesoblast 
which extends laterally to the edge of the area, and in each 
section following (z.e. towards the hind end of the primitive 
streak) the budding epiblast appears continually as a wider 
band until the greater part of the whole breadth of the 
epiblast plate is concerned in the production of mesoblast. 
A pit is seen in the epiblast almost at the front end of the 
primitive streak, and at this point a neurenteric canal will 
eventually be formed ; this structure, hitherto overlooked in 
mammalian embryos, is identical with the neurenteric canal 
found in other types of Vertebrata. 

The primitive streak grows relatively longer compared 
with the increase in size of the embryonic area, until in a 
vesicle, in which the latter measures: about 84 by ‘71 mm., 
the primitive streak reaches along it fully three parts of its 
length. It is very narrow in front, while behind it occupies 


GERMINAL LAYERS AND EARLY DEVELOPMENT OF MOLE. 113 


the whole breadth of the embryonic area. In sections of the 
region in front of the primitive streak there is present a 
layer of cells several rows thick immediately underlying the 
epiblast plate. In the seven anterior sections this layer is 
seen beneath the epiblast as a mass which cannot be resolved 
into hypoblast and mesoblast: for about three following 
sections, placed immediately in front of the primitive streak, 
the layer is clearly composed of (1) a layer of flattened hypo- 
blast below and (2) a layer of mesoblast above. The meso- 
blast in the axial line is thickened in the last two of these 
sections, and posteriorly joins the anterior wall of the 
neurenteric canal, while the hypoblast extends as a distinct 
layer below the anterior end of the primitive streak. It 
appears highly probable that the whole layer in the seven 
sections at the front end of the embryonic area is the hypo- 
blast originally present there engaged in the act of budding 
off mesoblast, as Balfour believes to be the case with regard 
to a similarly situated portion of the hypoblast in the chick 
(‘Comparative Embryology,’ vol. 11, p. 129 et. seg.). In 
the three following sections where distinct layers of mesoblast 
and hypoblast are found, the whole of the mesoblast, with 
the exception of the cells forming the central thickening in 
the second and third sections, has, I believe, a similar origin, 
and may be distinguished by the form and appearance of its 
cells from the mesoblast of the primitive streak. The 
mesoblast derived from the hypoblast may be called hypo- 
blastic mesoblast ; it joins the mesoblast of the primitive 
streak as the latter grows forward, and the two become 
indistinguishable. 

The primitive streak presents in section a similar appear- 
ance to that of the embryo last described; a groove—the 
primitive groove—is, however, present along its upper 
surface. The position of the future neurenteric canal is 
indicated by a pit in the epiblast asin the specimen described 
above. 

At a slightly later stage the embryonic area being 1°17 by 
‘81 mm., the condition of the layers is much the same. A 
neurenteric canal now perforates the whole thickness of the 
blastoderm at the front endof the primitive groove. The upper 
opening of this canal, which is longer than the lower opening, 
has the appearance of a slit with its anterior wall sloping 
obliquely backwards ; this wall is continuous with the thick- 
ening of mesoblast cells in the axial line which I described 
in the last stage. The first traces of the amnion are now 
visible as a fold of the epiblast round the whole circumference 
of the embryonic area; at the posterior end the folds of the 

10 


114 WALTER HEAPE. 


two sides meet to form a hood, covering the hinder part of 
the area, but anteriorly I have been unable to determine the 
extent of their growth. 

In the surface view of an embryonic area measuring ‘97 
by 79 mm. in diameter, a band of a lighter shade than the 
remainder is to be seen in the front part of the long axis of 
the area, its posterior end adjoining the anterior end of the 
primitive streak ; the latter occupies the hinder third of the 
area, and where it joins the light-coloured band a pit, the 
upper opening of the neurenteric canal, is distinctly to be 
seen surrounded by a dark rim. 

In transverse section the light-coloured band is seen to be 
caused by a diminution in thickness of the epiblast plate 
and of the mesoblast in the middle line. The epiblast of 
this region is bent inwards to form a groove, the medullary 
groove; it is wide and shallow throughout, and the cells 
forming it are not more than two rows deep, while the 
remainder of the epiblast plate, except at the extreme edge, 
is three cells deep. 

Anterior to the medullary groove a continuous layer hardly 
differentiated into mesoblast and hypoblast underlies the 
epiblast plate. In the region of the medullary groove an 
axial strip of the cells underlying the epiblast exhibits no 
division into hypoblast and mesoblast ; this portion, though 
partially separated from the lateral masses of mesoblast, is 
stil] connected with them, however, on each side by a narrow 
neck of cells, and is also directly continuous laterally with 
the hypoblast. ‘The lateral hypoblast is quite distinct from 
the superjacent lateral masses of mesoblast. The axial strip 
of cells underlying the medullary groove (thus shown to be 
continuous with both the mesoblast and the hypoblast) may 
be regarded as the commencing notochord. 

The neurenteric canal does not any longer perforate the 
blastoderm, its upper part alone remaining, which is sur- 
rounded by a thick mass of mesoblast, causing the dark rim 
seen in the surface view round the pit. Its anterior wall is 
connected with the axial mass of cells underlying the medul- 
lary groove, while its hind wall forms the front end of the 
primitive streak. 

The surface view of a somewhat older specimen, 1°5 mm. 
by ‘81 mm. diameter, shows the medullary groove relatively 
much longer and more clearly defined. Anteriorly it reaches 
nearly to the edge of the embryonic area. 

In section it is seen to be shallow at each end, but is much 
deeper and narrower towards the middle of the embryonic 
area. Its walls, at the anterior end, are but slightly less 


GERMINAL LAYERS AND EARLY DEVELOPMENT OF MOLE. 115 


thick than the remainder of the epiblast plate, but in the 
deeper part of the groove become considerably thinner. 
Where the groove is deepest the notochord and adjacent 
parts form a well-marked projection into the blastodermic 
cavity beneath. 

The rudimentary notochord has now extended beyond the 
anterior end of the medullary groove, and its relations to 
the adjacent layers are, for the most part, the same as in the 
previously described specimen. In front of the medullary 
groove it is composed of a single row of somewhat columnar 
cells, continuous laterally with both mesoblast and hypoblast ; 
below the anterior more shallow part of the medullary 
groove similar relations exist, towards its middle and deeper 
part, however, where the lateral mesoblast is commencing to 
form protovertebre, the notochordal cells, still in the form 
of a single row, are connected solely with the lateral 
plates of hypoblast, while further back, where the medullary 
groove becomes again more shallow, the cells of the noto- 
chord become more than one row deep, and are again con- 
tinuous both with the lateral mesoblast and hypoblast. The 
notochord, continually thickening towards its hinder ex- 
tremity, terminates by fusing with the anterior wall of the 
neurenteric canal. The latter structure is now open above 
on the floor of the posterior end of the medullary groove, 
and extends downwards into the cells beneath, though it no 
longer perforates the hypoblast, which is, however, somewhat 
involuted, and indistinguishably fused with the mesoblast in 
the median line. 

Briefly to recapitulate, I have attempted to show: 

(1.) The epiblast of the blastodermic vesicle owes its 
origin as well to the inner mass of segmentation spheres as 
to the outer layer of segments. It appears to originate in 
two ways: 

(a.) In an early stage of development (in the mole) 
probably by the cells of the inner mass being directly 
transformed into part of the wall of the blastodermic vesicle. 

(o.) In a later stage (mole and rabbit) by the transforma- 
tion of the rounded cells of the inner mass into a plate of 
columnar cells, which joins the part of the outer layer lying 
immediately above it to form the epiblast plate of the 
embryonic area. 

(2.) The mesoblast in the mole is formed in two portions : 

(a.) A larger portion which has its origin in the primitive 
streak. 

(6.) A smaller portion which is derived from the hypoblast 
situated in front of the primitive streak. 


116 WALTER HEAPE. 


I have been unable to distinguish where the latter, or 
hypoblastic mesoblast, comes into contact with the mesoblast 
of the primitive streak, and what part these respective layers 
take in the future development of the embryo. 

(3.) A neurenteric canal is present in the mole similar to 
that formed in other types of Vertebrata, first appearing as 
a pit at the anterior end of the primitive streak, while in 
later stages it perforates the floor of the hinder end of the 
medullary groove. 

I may here add that I have also found in a seven days’ 
rabbit embryo a rudimentary neurenteric canal in the form 
of a shallow pit in the epiblast at the front end of the 
primitive streak. 

(4.) The notochord is formed of an axial strip of cells, 
which underlies the epiblast of the medullary groove, and 
which either never becomes divided into mesoblast and 
hypoblast, or in which such a division, if it does take place 
(as appears not impossible), is very soon lost. This strip of 
cells is originally continuous laterally with both mesoblast 
and hypoblast, but as the lateral mesoblast becomes converted 
into definite vertebral plates the connection is lost. 

There can, I believe, be no doubt of the connection of the 
lateral hypoblast and mesoblast with the notochordal cells 
in the mole; in the rabbit I am inclined to believe that a 
similar connection is present, but my evidence on this point 
is not yet conclusive. 


PRINTED BY J. E. ADLARD. BAKIHOLOMEW CLOSE. 


A Renewep Stupy of the Germinat Layers of the 
Cuick. By F. M. Batrour, LL.D., F.R.S., 
Trinity College, Cambridge, and F. DricHron, 
B.A., St. Peter’s College. (With Plates VII, 
VIII and IX.) 


Tur formation of the germinal layers in the chick has 
been so often and so fully dealt with in recent years, that 
we consider some explanation to be required of the reasons which 
have induced us to add to the long list of memoirs on this 
subject. Our reasons are twofold. In the first place the prin- 
cipal results we have to record have already been briefly put 
forward in a ‘ Treatise on Comparative Embryology’ by one of 
us; and it seemed desirable that the data on which the con- 
clusions there stated rest should be recorded with greater detail 
than was possible in such a treatise. In the second place, our 
observations differ from those of most other investigators, in that 
they were primarily made with the object of testing a theory as 
to the nature of the primitive streak. As such they form acon- 
tribution to comparative embryology ; since our object has been 
to investigate how far the phenomena of the formation of the 
germinal layers in the chick admit of being compared with those 
of lower and less modified vertebrate types. 

We do not propose to weary the reader by giving a new 
version of the often told history of the views of various writers 
on the germinal layers in the chick, but our references to other 
investigators will be in the main confined to a comparison of our 
results with those of two embryologists, who have published 
their memoirs since our observations were made. One of them 
is L. Gerlach, who published a short memoir! in April last, and 
the other is C. Koller, who has published his memoir? still more 
recently. Both of them cover part of the ground of our investi- 


1 “Ueb. d. entodermale Entstehungswiese d. Chorda dorsalis,” ‘ Biol. 
Centralblatt,’ vol. i, Nos. 1 and 2. 
2.“ Untersuch. wb. d. Blatterbildung im Hiihnerkeim,” ‘ Archiv. f. 
mikr. Anat.,’ vol. xx, 1881. 
ll 


118 F. M, BALFOUR AND F. DEIGHTON. 


gations, and their results. are in many, though not in all points, 
in harmony with our own. Both of them, moreover, lay stress 
on certain features in the development which have escaped our 
attention. We desired to work over these points again, but 
various circumstances have prevented our doing so, and we have 


accordingly thought it best to publish our observations as they — 


stand, in spite of their incompleteness, merely indicating where 
the most important gaps occur. 

Our observations commence at a stage a few hours after hatch- 
ing, but before the appearance of the primitive streak. 

The area pellucida is at this stage nearly spherical. In it 
there is a large oval opaque patch, which is continued to the 
hinder border of the area. This opaque patch has received the 
name of the embryonic shield—a somewhat inappropriate name, 
since the structure in question has no very definite connection 
with the formation of the embryo. 

Koller describes, at this stage, in addition to the so-called 
embryonic shield, a sickle-shaped opaque appearance at the hinder 
border of the area pellucida. 

We have not made any fresh investigations for the purpose of 
testing Koller’s statements on this subject. 

Embryologists are in the main agreed as to the structure of 
the blastoderm at this stage. There is (Pl. VII, Ser. a, 1 and 
2) the epiblast above, forming a continuous layer, extending over 
the whole of the area opaca and area pellucida. In the former 
its cells are arranged as a single row, and are cubical or slightly 
flattened. In the latter the cells are more columnar, and form, in 
the centre especially, more or less clearly, a double row ; many of 
them, however, extend through the whole thickness of the layer. 

We have obtained evidence at this stage which tends to show 
that at its outer border the epiblast grows not merely by the 
division of its own cells, but also by the addition of cells derived 
from the yolk below. The epiblast has been observed to extend 
itself over the yolk by a similar process in many invertebrate 
forms. 

Below the epiblast there is placed, in the peripheral part of 
the area opaca, simply white yolk; while in a ring immediately 
outside and concentric with the area pellucida, there is a closely- 
packed layer of cells, known as the germinal wall. The 
constituent cells. of this wall are in part relatively small, of a 
spherical shape, with a distinct nucleus, and a granular and not 
very abundant protoplasm ; and in part large and spherical, filled 
up with highly refracting yolk particles of variable size, which 
usually render the nucleus (which is probably present) invisible 
(a, 1 and 2). This mass of cells rests, on its outer side, on a 
layer of white yolk. 


RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK, 119 


The sickle-shaped structure, visible in surface veins, is stated 
by Koller to be due to a special thickening of the germinal wall. 
We have not found this to be a very distinctly marked structure 
in our sections. 

In the region of the area pellucida there is placed below the 
epiblast a more or less irregular layer of cells. This layer is 
continuous, peripherally, with the germinal wall; and is com- 
posed of cells, which are distinguished both by their flattened 
or oval shape and more granular protoplasm from the epiblast- 
cells above, to which, moreover, they are by no means closely 
attached. Amongst these cells a few larger cells are usually 
present, similar to those we have already described as forming 
an important constituent of the germinal wall. 

We have figured two sections of a blastoderm of this age 
(Ser. 4, 1 and 2) mainly to show the arrangement of these cells. 
A large portion of them, considerably more flattened than the 
remainder, form a continuous membrane over the whole of the 
area pellucida, except usually for a small area in front, where 
- the membrane is more or less interrupted. ‘This layer is the 
hypoblast (Zy.). The remaining cells are interposed between this 
layer and the epiblast. In front of the embryonic shield there 
are either comparatively few or none of these cells present (Ser. a, 
1), but in the region of the embryonic shield they are very 
numerous (Ser. a, 2), and are, without doubt, the main cause 
of the opacity of this part of the area pellucida. These cells 
may be regarded as not yet completely differentiated segmen- 
tation spheres. 

In many blastoderms, not easily distinguishable in surface 
views from those which have the characters just described, the 
hypoblastic sheet is often much less completely differentiated, 
and we have met with other blastoderms, again, in which the 
hypoblastic sheet was completely established, except at the hinder 
part of the embryonic shield; where, in place of it and of the 
cells between it and the epiblast, there was only to be found a 
thickish layer of rounded cells, continuous behind with the 
germinal wall. 

In the next stage, of which we have examined surface views 
and sections, there is already a well-formed primitive streak. 

The area pellucida is still nearly spherical, the embryonic 
shield has either disappeared or become much less obvious, but 
there is present a dark linear streak, extending from the poste- 
rior border of the area pellucida towards the centre, its total 
length being about one third, or even less, of the diameter of the 
area. ‘This streak is the primitive streak. It enlarges con- 
siderably behind, where it joins the germinal wall. By Koller 
and Gerlach it is described as joining the sickle-shaped struc- 


120 F. M, BALFOUR AND F, DEIGHTON. 


ture already spoken of. We have in some instances found the 
posterior end of the primitive streak extending laterally in the 
form of two wings (Pl. IX, fig. 1). These extensions are, no 
doubt, the sickle ; but the figures given by Koller appear to us 
somewhat diagrammatic. One or two of the figures of early 
primitive streaks in the sparrow, given by Kupffer and Benecke,} 
correspond more closely with what we have found, except that 
in these figures the primitive streak does not reach the end of 
the area pellucida, which it certainly usually does at this early 
stage in the chick. 

Sections through the area pellucida (Pl. VII, ser. B and c) 
give the following results as to the structure of its constituent 
parts. 

The epiblast cells have undergone division to a considerable 
extent, and in the middle part, especially, are decidedly more 
columnar than at an earlier stage, and distinctly divided into 
two rows, the nuclei of which form two more or less distinct 
layers. 

In the region in front of the primitive streak the cells of the 
lower part of the blastoderm have arranged themselves as a de- 
finite layer, the cells of which are not so flat as is the case with 
the hypoblast cells of the posterior part of the blastoderm, and 
in the older specimens of this stage they are very decidedly 
more columnar than in the younger specimens. 

The primitive streak is however the most interesting structure 
in the area pellucida at this stage. 

The feature which most obviously strikes the observer in 
transverse sections through it is the fact, proved by Kolliker, that 
it is mainly due to a proliferation of the epiblast cells along an 
axial streak, which, roughly speaking, corresponds with the dark 
line visible in surface views. In the youngest specimens and at 
the front end of the primitive streak, the proliferated cells do not 
extend laterally beyond the region of their origin, but in the 
older specimens they have a considerable lateral extension. 

The hypoblast can, in most instances, be traced as a distinct 
layer underneath the primitive streak, although it is usually less 
easy to follow it in that region than elsewhere, and in some 
cases it can hardly be distinctly separated from the superjacent 
cells. 

The cells undoubtedly formed by a proliferation of the epi- 
blast, form a compact mass extending downwards towards the 
hypoblast ; but between this mass and the hypoblast there are 
almost always present along the whole length of the primitive 


streak a number of cells, more or less loosely arranged, and 


1 « Photogramme d. Ontogenie d. Vogel.” Nova Acta. K. Leop. Carol, 
* Deutschen Akad. d. Naturfor.,’ Bd. x, 41, 1879. 


laa ea 


RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK, 121 


decidedly more granular than the proliferated cells. Amongst 
these loosely arranged cells there are to be found a certain number 
of large spherical cells filled with yolk granules. Sometimes 
‘these cells are entirely confined to the region of primitive streak, 
at other times they are continuous laterally with cells irregularly 
scattered between the hypoblast and epiblast (Ser. c, 2), which are 
clearly the remuants of the undifferentiated _cells of the embryonic 
shield. The junction between these cells and the cells of the primi- 
tive streak derived from the epiblast is often obscure, the two sets 
of cells becoming partially intermingled. The facility with which 
the cells we have just spoken of can be recognised varies more- 
over greatly in different instances. In some cases they are very 
obvious (Ser. c), while in other cases they can only be dis- 
tinguished by a careful examination of good sections. 

The cells of the primitive streak between the epiblast and the 
hypoblast are without doubt mesoblastic, and constitute the first 
portion of the mesoblast which is established. -The section of 
these cells attached to the epiblast, in our opinion, clearly ori- 
ginates from the epiblast; while the looser cells adjoining the 
hypoblast must, it appears to us, be admitted to have their origin 
in the indifferent cells of the embryonic shield, placed between the 
epiblast and the hypoblast, and also very probably in a distinct 
proliferation from the hypoblast below the primitive streak. 

Posteriorly the breadth of the streak of epiblast which buds | 
off the cells of the primitive streak widens considerably, and in 
the case of the blastoderm with the earliest primitive streaks 
extends into the region of the area opaca. The widening of the 
primitive streak behind is shown in Ser. B, 3 ; Ser. c, 2; and Ser. 
gE, 4. Where very marked it gives rise to the sickle-shaped 
appearance upon which so much stress has been laid by Koller 
and Gerlach. In the case of one of the youngest of our blasto- 
derms of this stage in which we found in surface views (Pl. IX, 
fig. 1) a very well-marked sickle-shaped appearance at the hind 
end of the primitive streak, the appearance was caused, as ‘is 
clearly brought out by our sections, by a thickening of the hypo- 
blast of the germinal wall. 

There is a short gap in our observations between the stage 
with a young primitive streak and the first described stage in 
which no such structure is present. ‘This gap has been filled 
up both by Gerlach and Koller. 

Gerlach states that during this period a small portion of the 
epiblast, within the region of the area opaca, but close to the 
posterior border of the area pellucida, becomes thickened by a 
proliferation of its cells. This portion gradually grows out- 
wards laterally, forming in this way a sickle-shaped structure. 
From the middle of this sickle a process next grows forward into 


122 F, M. BALFOUR AND F, DEIGHTON, 


the area pellucida. This process is the primitive streak, and it 
is formed, like the sickle, of proliferating epiblast cells. 

Koller! described the sickle and the growth forwards from it 
of the primitive streak in surface views somewhat before Gerlach ; 
and in his later memoir has entered with considerable detail 
into the part played by the various layers in the formation of 
this structure. 

He believes, as already mentioned, that the sickle-shaped 
structure, which appears according to him at an earlier stage 
than is admitted by Gerlach, is in the first instance due to a 
thickening of the hypoblast. At a later stage he finds that the 
epiblast in the centre of the sickle becomes thickened, and that a 
groove makes its appearance in this thickening which he calls 
the “Sichel-rinne.” ‘This groove is identical with that first 
described by Kupffer and Benecke* in the sparrow and fowl. 
We have never, however, found very clear indications of it in 
our sections. 

In the next stage, Koller states that, in the region immediately 
in front of the ‘‘ Sichel-rinne,”’ a prominence appears which he 
calls the Sichelknopf, and from this a process grows forwards 
which constitutes the primitive streak. This structure is in 
main derived from a proliferation of epiblast cells, but Koller 
admits that some of the cells just above the hypoblast in the region 
of the Sichelknopf are probably derived from the hypoblast. 
Since these cells form part of the mesoblast it is obvious that 
Koller’s views on the origin of the mesoblast of the primitive 
streak closely approach those which we have put forward. 

The primitive streak starting, as we have seen, at the hinder 
border of the area pellucida, soon elongates till it eventually 
occupies at least two thirds of the length of the area. As 
Koller (loc. cit.) has stated this can only be supposed to happen 
in one of two ways, viz. either by a progression forward of the 
region of epiblast budding off mesoblast, or by an interstitial 
growth of area of budding epiblast. Koller adopts the second 
of these alternatives, but we cannot follow him in doing so. 
The simplest method of testing the point is by measuring the 
distance between the front end of the primitive streak and the 
front border of the area pellucida at different stages of growth of 
the primitive streak. If this distance diminishes with the elon- 
gation of the primitive streak then clearly the second of the two 
alternatives is out of the question. 

We have made measurements to test this point, and find that 
the diminution of the space between the front end of the 

1 “Beitr. z. Kenntniss d. Hiihnerkeims im Beginne d. Bebritung,” 
‘ Sitz. d. k. Akad. Wiss.,’ iv Abth., 1879. 

* «Die erste Entwick, an Hier d. Reptilien.’ Konigsberg, 1878. 


RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 1238 


primitive streak and the anterior border of the area pellucida is 
very marked up to the period in which the medullary plate first 
becomes established. We can further point in support of our 
view to the fact that the extent of the growth lateralwards of the 
mesoblast from the sides of the primitive streak is always less in 
front than behind; which would seem to indicate that the front 
part of the streak is the part formed latest. Our view as to the 
elongation of the primitive streak appears to be that adopted by 
Gerlach. 

Our next stage includes roughly the period commencing 
slightly before the first formation of a groove along the primi- 
tive streak, known as the primitive groove, and terminating 
immediately before the first trace of the notochord makes its 
appearance. After the close of the last stage the primitive 
streak gradually elongates, till it occupies fully two thirds of the 
diameter of the area pellucida. ‘The latter structure also soon 
changes its form from a circular to an oval, and finally becomes 
pyriform with the narrow end behind, while the primitive streak 
occupying two thirds of its long axis becomes in most instances 
marked by a light linear band along the centre, which constitutes 
the primitive groove. 

In surface views the primitive streak often appears to stop 
short of the hinder border of the area pellucida. 

During the period in which the external changes, which we 
have thus briefly described, take place in the area pellucida, 
great modifications are effected in the characters of the germinal 
layers. The most important of these concern the region in 
front of the primitive streak ; but they will be better understood 
if we commence our description with the changes in the primitive 
streak itself. 

In the older embryos belonging to our last stage we pointed 
out that the mesoblast of the primitive streak was commencing 
to extend outwards from the median line in the form of two 
lateral sheets. This growth of the mesoblast is continued rapidly 
during the present stage, so that during the latter part of it any 
section through the primitive streak has approximately the cha- 
racters of Ser. 1, 5. 

The mesoblast is attached in the median line to the epiblast. 
Laterally it extends outwards to the edge of the area pellucida, 
and in older embryos may even form a thickening beyond the edge 
(fig. @). Beneath the denser part of the mesoblast, and attached 
to the epiblast, a portion composed of stellate cells may in the 
majority of instances be recognised, especially in the front part 
of the primitive streak. We believe these stellate cells to be in 
the main directly derived from the more granular cells of the 
previous stage. The hypoblast forms a sheet of flattened cells, 


124 F. M. BALFOUR AND F. DEIGHTON. 


which can be distinctly traced for the whole breadth of the area 
pellucida, though closely attached to the mesoblast above. 

In sections we find that the primitive streak extends back to 
the border of the area pellucida, and even for some distance 
beyond. ‘The attachment to the epiblast is wider behind; but 
the thickness of the mesoblast is not usually greater in the 
median line than it is laterally, and for this reason probably the 
posterior part of the streak fails to show up in surface views. 
The thinning out of the median portion of the mesoblast of the 
primitive streak is shown in a longitudinal section of a duck’s 
blastoderm of this stage (fig. p). The same figure also shows 
that the hypoblastic sheet becomes somewhat thicker behind, 
and more independent of the parts above. 

A careful study of the peripheral part of the area pellucida, in 
the region of the primitive streak, in older embryos of this stage, 
shows that the hypoblast is here thickened, and that its upper 
part, i.e. that adjoining the mesoblast, is often formed of stellate 
cells, many of which give the impression of being in the act of 
passing into the mesoblast above. At a later stage the meso- 
blast of the vascular area undoubtedly receives accessions of cells 
from the yolk below; so that we see no grounds for mistrusting 
the appearances just spoken of, or for doubting that they are to 
be interpreted in the sense suggested. 

We have already stated that during the greater part of the 
present stage a groove, known as the primitive groove, is to be 
found along the dorsal median line of the primitive streak. 

The extent to which this groove is developed appears to be 
subject to very great variation. On the average it is, perhaps, 
slightly deeper than it is represented in Ser.1, 5. Insome cases 
it is very much deeper. One of the latter is represented in 
fig. a. Jt has here the appearance of a narrow slit, and 
sections of it give the impression of the mesoblast originating 
from the lips of a fold; in fact, the whole structure appears like 
a linear blastopore, from the sides of which the mesoblast is 
growing out; and this as we conceive actually to be the true inter- 
pretation of the structure. Other cases occur in which the 
primitive groove is wholly deficient, or at the utmost represented 
by a shallow depression along the median axial line of a short 
posterior part of the primitive streak. 

We may now pass to the consideration of the part of the area 
pellucida in front of the primitive streak. 

We called attention to a change in the character of the hypo- 
blast cells of this region as taking place at the end of the last 
stage. During the very early part of this stage the change in 
the character of these cells becomes very pronounced. 

What we consider to be our earliest stage in this change we 


RENEWED STUDY OF GERMINAL LAYERS O¥ THE CHICK, 125 


have only so far met with in the duck, und we have figured a longi- 
tudinal and median section to show it (Pl. VII, fig. pv). The 
hypoblast (Ay) has become a thick layer of somewhat cubical cells 
several rows deep. These cells, especially in front, are characterised 
by their numerous yolk spherules, and give the impression that 
part of the area pellucida has been, so to speak, reclaimed from the 
tarea opaca. Posteriorly, at the front end of the primitive streak, 
the thick layer of hypoblast, instead of being continuous with the 
flattened hypoblast under the primitive streak, falls, in the axial 
line, into the mesoblast of the primitive streak (Pl. VII, fig. p). 

In a slightly later stage, of which we have specimens both of 
the duck and chick, but have only figured selected sections of a 
chick series, still further changes have been effected in the con- 
stitution of the hypoblast (Pl. VIII, Ser. n, 1 and 2). 

Near the front border of the area pellucida (1) it has the 
general characters of the hypoblast of the duck’s blastoderm just 
described. Slightly further back the cells of the hypoblast have 
become differentiated into stellate cells several rows deep, which 
can hardly be resolved in the axial line into hypoblast and meso- 
blast, though one can fancy that in places, especially laterally, 
they are partially differentiated into two layers. The axial sheet 
of stellate cells is continuous laterally with cubical hypoblast 
cells. 

As the primitive streak is approached an axial prolongation 
forwards of the rounded and _ closely-packed mesoblastic 
elements of the primitive streak is next met with, and at the 
front end of the primitive streak, where this prolongation unites 
with the epiblast, it also becomes continuous with the stellate 
cells just spoken of. In fact, close to the end of the primitive 
streak it becomes difficult to say which mesoblast cells are directly 
derived from the primitive layer of hypoblast in front of the primi- 
tive streak, and which from the forward growth of the mesoblast 
of the primitive streak. There is, in fact, as in the earlier stage, 
a fusion of the layers at this point. 

Sections of a slightly older chick blastoderm are represented 
m Pl. VII, Ser. 1, 1, 2, 3, 4 and 5. 

Nearly the whole of the hypoblast in front of the primitive 
streak has now undergone a differentiation into stellate cells. 
In the second section the products of the differentiation of this 
layer form a distinct mesoblast and hypoblast laterally, while in 
the median line they can hardly be divided into two distinct 
layers. 

5" a section slightly further back the same is true, except that 
we have here, in the axial line above the stellate cells, rounded ele- 
ments derived from a forward prolongation of the cells of the 
primitive streak. In the next section figured, passing through 


126 F, M. BALFOUR AND F, DEIGHTON. 


the front end of the primitive streak, the axial cells have become 
continuous with the axial mesoblast of the primitive streak, 
while below there is an independent sheet of flattened hypoblast 
cells. 

The general result of our observations on the part of the 
blastoderm in front of the primitive streak during this stage is 
to show that the primitive hypoblast of this region undergoes 
considerable changes, including a multiplication of its cells ; and 
that these changes result in its becoming differentiated on each 
side of the middle line, with more or less distinctness, into (1) a 
hypoblastic sheet below, formed of a single row of flattened cells, 
and (2) a mesoblast plate above formed of stellate cells, while in 
the middle line there is a strip of stellate cells in which there is 
no distinct differentiation into two layers. 

Since the region in which these changes take place is that in 
which the medullary plate becomes subsequently formed, the 
lateral parts of the mesoblast plate are clearly the permanent 
lateral plates of the trunk, from which the mesoblastic somites, &., 
become subsequently formed ; so that the main part of the meso- 
blast of the trunk is not directly derived from the primitive streak. 

Before leaving this stage we would call attention to the presence, 
in one of our blastoderms of this stage, ofa deep pit at the junc- 
tion of the primitive streak with the region in front of it (Pl. VIII, 
Ser. F, l and 2). Such a pit is unusual, but we think it may 
be regarded as an exceptionally early commencement of that 
most variable structure in the chick, the neurenteric canal. 

The next and last stage we have>to deal with is that during 
which the first trace of the notochord and of the medullary plate 
make their appearance. 

In surface views this stage is marked by the appearance of a 
faint dark line, extending forwards, from the front end of the 
primitive streak, to a fold, which has in the mean time made its 
appearance near the front end of the area pellucida, and con- 
stitutes the head fold. 

Pl. IX, Ser. K, represents a series of sections through a blas- 
toderm of this stage, which have been selected to illustrate the 
inode of formation of the notochord. 

In a section immediately behind the head fold the median part 
of the epiblast is thicker than the lateral parts, forming the first 
indication of a medullary plate (Ser. x, 1). Below the median 
line of the epiblast is a small cord of cells, not divided into two 
layers, but continuous laterally, both with the hypoblast and 
mesoblast, which are still more distinctly separated than in 
the previous stage. 

A section or so further back (Ser. k, 2) the axial cord, which 
we need scarcely say is the rudiment of the notochord, is thicker, 


RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 127 


and causes a slight projection in the epiblast above. It is, as 
before, continuous laterally, both with the mesoblast and with 
the hypoblast. The medullary plate is more distinct, and a 
shallow but unmistakable medullary groove has made its 
appearance. 

As we approach the front end of the primitive streak the 
notochord becomes (Ser. k, 3) very much more prominent, 
though retaining the same relation to the germinal layers as in 
front. 

In the section immediately behind (Ser. x, 4) the convex upper 
surface of the notochord has become continuous with the epi- 
blast for a very small region. ‘The section, in fact, traverses the 
front end of the primitive streak. 

In the next section the attachment between the epiblast and 
the cells below becomes considerably wider. It will be noticed 
that this part of the primitive streak is placed on the floor of the 
wide medullary groove, and there forms a prominence known as 
the anterior swelling of the primitive streak. 

It will further be noticed that in the two sections passing 
through the primitive streak, the hypoblast, instead of simply 
becoming continuous with the axial thickening of the cells, as in 
front, forms a more or less imperfect layer underneath it. This 
layer becomes in the sections following still more definite, and 
forms part of the continuous layer of hypoblast present in the 
region of the primitive streak. 

A comparison of this stage with the previous one shows very 
clearly that the notochord is formed out of the median plate 
of cells of the earlier stage, which was not divided into mesoblast 
and hypoblast, together with the short column of cells which 
grew forwards from the primitive streak. 

The notochord, from its mode of origin, is necessarily con- 
tinuous behind with the axial cells of the primitive streak. 

The sections immediately behind the last we have represented 
show a rudiment of the neurenteric canal of the same form as 
that first figured by Gasser, viz. a pit perforating the epiblast with 
a great mass of rounded cells projecting upwards through it. 


The observations just recorded practically deal with two much 
disputed points in the ontogeny of birds, viz. the origin of the 
mesoblast and the origin of the notochord. 

With reference to the first of these our results are briefly as 
follows : 

The first part of the mesoblast to be formed is that which 
arises in connection with the primitive streak. This part is in 
the main formed by a proliferation from an axial strip of the 
epiblast along the line of the primitive streak, but in part also from 


128°: F. M. BALFOUR AND F. DEIGHTON, 


a simultaneous differentiation of hypoblast cells also along the 
axial line of the primitive streak. The two parts of the meso- 
blast so formed become subsequently indistinguishable. The 
second part of the mesoblast to be formed is that which gives 
rise to the lateral plates of mesoblast of the head and trunk 
of the embryo. This part appears as two plates—one on each 
side of the middle line—which arise by direct differentiation 
from the hypoblast in front of the primitive streak. They are 
continuous behind with the lateral wings of mesoblast which grow 
out from the primitive streak, and on their inner side are also 
at first continuous with the cells which form the notochord. 

In addition to the parts of mesoblast, formed as just described, 
the mesoblast of the vascular area is in a large measure developed 
by a direct formation of cells round the nuclei of the germinal wall. 

The mesoblast formed in connection with the primitive streak 
gives rise in part to the mesoblast of the allantois, and ventral part 
of the tail of the embryo (?), and in part to the vascular struc- 
tures found in the area pellucida. | 

With reference to the formation of the mesoblast of the primi- 
tive streak, our conclusions are practically in harmony with those 
of Koller; except that Koller is inclined to minimise the share 
taken by the hypoblast in the formation of the mesoblast of the 
primitive streak. 

Gerlach, with reference to the formation of this part of the 
mesoblast, adopts the now generally accepted view of Kolliker, 
according to which the whole of the mesoblast of the primitive 
streak is derived from the epiblast. 

As to the derivation of the lateral plates of mesoblast of the 
trunk from the hypoblast of the anterior part of the primitive 
streak, our general result is in complete harmony with Gerlach’s 
results, although in our accounts of the details of the process we 
differ in some not unimportant particulars. 

As to the origin of the notochord, our main result is that this 
structure is formed as an actual thickening of the primitive 
hypoblast of the anterior part of the area pellucida. We find 
that it unites posteriorly with a forward growth of the axial 
tissue of the primitive streak, while it is laterally continuous, at 
first, both with the mesoblast of the lateral plates and with the 
hypoblast. At a later period its connection with the mesoblast 
is severed, while the hypoblast becomes differentiated as a con- 
tinuous layer below it. 

As to the hypoblastic origin of the notochord, we are again in 
complete accord with Gerlach; but we differ from him in admitting 
that the notochord is continuous posteriorly with the axial tissue 
of the primitive streak, and also at first continuous with the 
lateral plates of mesoblast. 


RENEWED STUDY OF GERMINAL LAYERS OF THE CHICK. 129 


The account we have given of the formation of the mesoblast 
may appear to the reader somewhat fantastic, and on that account 
not very credible. We believe, however, that if the view which 
has been elsewhere urged by one of us, that the primitive streak 
is the homologue of the blastopore of the lower vertebrates is 
accepted, the features we have described receive an adequate 
explanation. 

The growth outwards of part of the mesoblast from the axial 
line of the primitive streak is a repetition of the well-known 
growth from the lips of the blastopore. It might have been an- 
ticipated that all the layers would fuse along the line of the 
primitive streak, and that the hypoblast as well as part of the 
mesoblast would grow out from it. There is, however, clearly 
a precocious formation of the hypoblast ; but the formation of the 
mesoblast of the primitive streak, partly from the epiblast and 
partly from the hypoblast, is satisfactorily explained by regarding 
the whole structure as the blastopore. The two parts of the 
mesoblast subsequently become indistinguishable, and their 
difference in origin is, on the above view, to be regarded as 
simply due toa difference of position, and not as having a deeper 
significance. 

The differentiation of the lateral plates of mesoblast of the 
trunk directly from the hypoblast is again a fundamental feature 
of vertebrate embryology, occurring in all types from Amphioxus 
upwards, the meaning of which has been fully dealt with in the 
‘Treatise on Comparative Embryology’ by one of us. Lastly, 
the formation of the notochord from the hypoblast is the typical 
vertebrate mode of formation of this organ, while the fusion of 
the layers at the front end of the primitive streak is the 
universal fusion of the layers at the dorsal lip of the blastopore, 
which is so well known in the lower vertebrate types. 


EXPLANATION OF PLATES VII, VIII & IX, 


Illustrating Messrs. Balfour and Deighton’s Paper on “A 
Renewed Study of the Germinal Layers of the Chick.” 


N.B.—The series of sections are in all cases numbered from before 
backwards. 


List of Reference Letters. 


ep. Hpiblast. fy. Hypoblast. mm. Mesoblast. gr. Germinal wall. 
yk. Yolk of germinal wall. pus. Primitive streak. pr. g. Primitive 
groove. ch. Notochord. a. p. Area pellucida. o. p. Area opaca. 


PLATE VII. 


Serizs A, 1 and 2.—Sections through the blastoderm before the appearance 
of primitive streak. 

1. Section through anterior part of area pellucida in front of em- 
bryonic shield. The hypoblast here forms an imperfect layer. The 
figure represents about half the section. 2. Section through same blas- 
toderm, in the region of the embryonic shield. Between the epiblast 
and hypoblast are a number of undifferentiated cells. The figure repre- 
sents considerably more than half the section. 


Serizs B, 1, 2 and 3.—Sections through a blastoderm with a very young 
primitive streak. 

1. Section through the anterior part of the area pellucida in front 
of the primitive streak. 2. Section through about the middle of the 
primitive streak. 3. Section through the posterior part of the primi- 
‘tive streak. 


Series C, 1 and 2.—Sections through a blastoderm with a young primitive 
streak. 

1. Section through the front end of the primitive streak. 2. Section 
through the primitive streak, somewhat behind 1. Both figures 
show very clearly the difference in character between the cells of the 
epiblastic mesoblast of the primitive streak, and the more granular 
cells of the mesoblast derived from the hypoblast. 


Fie. D.—Longitudinal section through the axial line of the primitive 
streak, and the part of the blastoderm in front of it, of an embryo 
duck with a well-developed primitive streak. 


Serizs E,' 1, 2, 3 and 4.—Sections through blastoderm with a primitive 
streak, towards the end of the first stage. 

1. Section through the anterior part of the area pellucida. 2. Sec. 
tion a little way behind 1 showing a forward growth of mesoblast from 
the primitive streak. 3. Section through primitive streak. 4. Section 
through posterior part of primitive streak, showing the great widening 
of primitive streak behind. 


1 3 and 4 of this series are placed on Plate VIII. 


EXPLANATION OF PLATES VII, VII & 1X—continued. 


PLATE VIII. 


Series F, 1 and 2.—Sections through a blastoderm with primitive groove. 

1. Section showing a deep pit in front of primitive streak, probably 

an early indication of the neurenteric canal. 2. Section immediately 
following 1. 


Fie. G.—Section through blastoderm with well developed primitive streak, 
showing an exceptionally deep slit-like primitive groove. 


Series H, 1 and 2.—Sections through a blastoderm with a fully-developed 
primitive streak. 

1. Section through the anterior part of area pellucida, showing the 
cubical granular hypoblast cells in this region. 2. Section slightly 
behind 1, showing the primitive hypoblast cells differentiated into stellate 
cells, which can hardly be resolved in the middle line into hypoblast 
and mesoblast. 


Serizs J, 1, 2,3, 4 and 5.—Sections through blastoderm somewhat older 
than Series H. 
1. Section through area pellucida well in front of primitive streak. 
2. Section through area pellucida just in front of primitive streak. 
3. Section through the front end of primitive streak. 4, Section slightly 
behind 3. 5. Section slightly behind 4. 


PLATE IX, 


Series K, 1, 2, 3, 4 and 5.—Sections through a blastoderm in which the 
first trace of notochord and medullary groove have made their appear- 
ance. Rather more than half the section is represented in each figure, 
but the right half is represented in 1 and 3, and the left in 2 and 4. 

1. Section through notochord immediately behind the head-fold. 
2. Section showing medullary groove a little behind 1. 3. Section 
just in front of the primitive streak. 4 and 5. Sections through 
the front end of the primitive streak. 


Fic. L.—Surface view of blastoderm with a very young primitive streak. 


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